Title says it all, or nearly all: it’s free.
ps
What’s the catch?
Any downside to upgrading from Lion to Mavericks?
There isn’t much of a catch if you are upgrading from Lion. Of course the typical “Dot Zero” operating system rule of thumb applies: don’t do it unless you are prepared for the worst. If you have a good back up, then go for it. The battery savings, if that matters, may mean you can leave the power adapter home. Beyond that there is little reason though. This update is free for a reason in my opinion.
Noted, thanks AmberV 
I will report briefly on my installation experience.
Upgrading from Mountain Lion to Mavericks.
On a 2008-era iMac with 4 gb of RAM installed.
On a fast (15 mbs) broadband system.
Download time from Apple Store: over 60 minutes
Installation time: also over 60 minutes
Optimizing system: another 60 minutes or more
So that’s over 3 hours, and at the moment
I see few of the “new features” that I will actually use.
Maps and iBooks? I will stick to Google and Kindle.
Tagging of files? I like my own filing system.
Tabbing of Finder? long available in Chrome.
Notifications? I find them annoying.
Safari update: I use the faster, more versatile Chrome
iCloud keychain: 1Password and Chrome handle this
Calendar: it was OK, what about that miserable ***** Contacts??
Increasingly, my baseline is Chrome.
The only reason I stick with Apple is Scrivener.
And I’m hoping that one day we’ll have a browser version.
Free of platform, synching, and updating woes.
You browse your local files system with a web browser?
Yes. Looks like a ftp server listing. Been available in all browsers since … when did lynx show up? Lets just call it forever.
I’m not at all surprised that you can browse your filesystem with a browser, but I am surprised that one would want to do so. 
Just doesn’t seem like the ideal tool for the job. Then again, the Finder hasn’t been all that ideal (without enhancements like TotalFinder at least).
Tabbing of Finder…
I didn’t express myself well.
I meant that the Chrome interface is always tabbed
Including when I browse files on Google Drive
In the Finder, I’ve tried every kind of 3rd party assist
from the late lamented On Location,
in which you could browse and view file contents
to Yep, Leap, etc
Certainly a tabbed Finder is better than multiple windows
And it’s only taken 10-12 years to produce the improvement.
I still use one window = one folder, like the way Mac was meant to be used.
I find tabbed windows in the Finder useful for the way I work. Usually, I work on a main project, but need several references to other/previous projects. Having these latter in the same window will help me clean up the overly cluttered workspace.
I guess I will end up with just a few windows: Technical writing, Creative writing, University, Music composition, Movie making. Much cleaner.
Paolo
Hey! So do I. How good is that? Dead good init? 
Vic 1w=1f-k 
Been using Mavericks on an old 2008 iMac since its release. Noticeably faster, especially Safari, and much better memory management. iWork and iLife updated FOC. Very pleased.
I may step up my plans to upgrade, seeing that the battery life is dramatically improved by it. That was the most important factor to me when I got my 13" Air after pining for the 11" model. Anything that gets me closer to the theoretical “all-day” usage without needing to plug in is a priority for me.
Finally we left the kingdom of cats and drifted into the kingdom of Tom Cruise characters.
Young Master Sin, you can never leave the kingdom of cats! We’re everywhere. 
Fluff
Mavericks: how all cats view themselves.
Fluff concurs.
I have a mixed response to Mavericks. I have not seen evidence (yet) of extended battery life, but that may come. At times, however, my computer (late 2008 MBP) seems slower and less responsive. 
On the plus side, the price was unexpected and unbeatable. Installation was simple and went without hitch - really couldn’t have been easier. And, generally, I like the improved links with iOS. Some of the new features are good and I am especially looking forward to experiencing how Mavericks deals with multiple monitors. I’m still adjusting to the new visuals (e.g. Calendar is good, Top Sites in Safari not so good, but Contacts is finally usable again and is an enormous improvement). I’ve had some problems with Mail, especially smart mailboxes not recognising content or showing incorrect unread mail counts. Seems to be improving over time, but the initial experience left me distinctly underwhelmed.
I like the idea of iCloud Keychain, but haven’t yet got it to work for me. iBooks on the Mac is great. Just reread a couple of chapter of Made Man in full screen view. Almost as good as my iPad.
I like the ability to add travel time to Calendar appointments, but am frustrated and annoyed that that information is then not viewable on iOS (where, logically, it is even more salient). What’s worse, editing an appointment on iOS deletes all of the travel meta data (so much for improved links with iOS). Further, it is incomprehensible why you can’t click on an address in Calendar and have it show on in the new Maps application - if you can do this on an iPhone, why not a Mac? Nor can you adjust the route that Maps selects (something Google Maps has been capable of for years).
In Finder, I like the ability to add tags and think the implementation is reasonable for general use (although it won’t satisfy power users). The visual display of tag colours is awful - instead of the entire text bubble for the name being coloured, now there is a little coloured dot beside the name. UGLY. Yet this is (almost) compensated for by the ability to use tabs in Finder which is nicely done and much appreciated. In fact this feature, along with iBooks, is almost enough justify the upgrade.
I have some misgivings about the new iWorks apps. Unfortunately I updated my iOS apps before I discovered that they used a new format and I had to update my Mac apps in order to continue sharing files with them. Would have been nice if Apple explained this before I updated. At least Apple had the good grace to leave the previous Mac versions available on my Mac when I updated them so I can continue to use the originals if needed (just need to remember to create copies of any files before I update them). Making apps less intuitive and harder while accompanied by loss of features is an interesting approach (although similar to the update to Final Cut X which, I’m told, is starting to pay off now but was a tough sell to start with). I’ll trial these apps further, but so far, not impressed.
I suspect that Mavericks on a newer machine would be lovely. I know I’m pushing the specs a little and really should be impressed it runs on my machine at all (actually, I am). Despite the tone of the above (I’m tired, been a long day) overall I’d come down in favour of Mavericks. But it’s a closer call than previous upgrades.
THE GOOD
(1) The gray linen is gone.
(2) Multiple monitor setups works better. (Menu for each screen - No hideous gray linen black out on secondary monitors when one goes full screen.)
(3) Safari 7 implements multiple process much better - less crashing.
(4) iBooks on the Mac (what an oversight)
(5) Compressed memory for Macs that don’t have a lot of RAM and were slammed with thrashing from swap files.
(6) Better energy consumption.
(7) Many small bug fixes and corrections.
THE BAD
(1) Tagging is confusing at times. Needs a major UI improvement. Some instances are confusing. Limited colors is like a throw back to early days of the finder (pre OSX) Hey guys for a computer used in the “arts” more than any other Apple still feels there are only 7 colors in the world!
(2) Mounted drives (shares) don’t show up in the sidebar like they used to. You have to go to main Computer that is sharing then connect, then select share, even if you already have a share mounted on your desktop when opening from an open dialog box.
(3) MAPS still sucks and is not very accurate. Pretty UI but I don’t trust MAPS yet. Feels like mapquest in the very first year. Get ready to wind up in a cornfield or the wrong side of town.
(4) Not a lot of real noticeable changes above the hood that is unless you like a OSX-iOS and you are a notification junkie.
THE UGLY
Contacts and some of the other UI “tweaks” looks like something from system 6. Bland is not even the word I would use. More like Asylum moronic. New Maverick programs with new UI but the old ICONs on some. Pretty much looks rushed. The os still feels Schizophrenic when it comes to icons and UI choices. Apple has gotten down right lazy. I guess this happened because of course iOS 7 wasn’t going to reach its release date so they pulled the engineers from OSX Mavericks to help finish iOS 7. It shows. Terrible job this time around. Looks like an early Beta not a finished product. Mavericks is the new naming scheme. Famous places in California… bland once again.
The reality is Mavericks is free and should be. It is less dramatic then the upgrade from Leopard to Snow Leopard. The name should have been Mountain Lioness since it is mainly some bug fixes and some poorly rushed UI tweaks that carry the user towards a big fat train wreck.
So far the worst “upgrade” apple has released.
Thinking more of the lines of (Bug fixes) with some half assed unfinished UI experiments.
If it wasn’t for the bug fixes I would have never upgraded since in reality its two steps back in functionality, UI and general Use.
That is unless you are 14 and all you ever do is listen to iTunes, take pictures with an iPhone (photo stream) and tweet announcements about your new 5 minute ago face book post about the color of your last bowel movement.
This is the first time I have ever said this but actually looking at Ubuntu or W7 alternatives. Apple appears to be focused on iOS devices and gimmicky games and letting the work horses collect dust and shrivel on the vine.