Well, because I pay for developer access, I have been using Leopard for a few months now, and for the past two or three months it has been mostly feature-complete. I have to say that I love it. I have just finished downloading the GM version (Apple seeded it to developers at the same time as releasing it in stores) and will install it as my primary OS later today. Though actually, I've been using it as my primary OS for a while.
For a start, there is nothing that was better in Tiger. At first I didn't like the new darker interface, so I reckon some folk will complain about that to begin with, but now I much, much prefer it. Tiger looks washed out by comparison. It's also great to have a unified interface at last (excepting the iApps... I have no idea why they have completely different scroll bars, but there you go).
Superb. At last the Finder feels modern and a joy to use. I hated the old, brushed-metal thing. And Cover Flow, which at first I thought was just a gimmick, is absolutely fantastic for quickly browsing through image and movie files. In conjunction with QuickLook, it's great for browsing through pretty much any type of file. (Scrivener, for instance, will show the synopses of the draft in QuickLook.)
Much, much faster.
Hmm... I'm not 100% sure there is one. To me, Leopard feels like a much more mature version of Tiger. Everything that was great about Tiger has been kept and the stuff that was annoying on Tiger - mainly the lack of a consistent UI - has been fixed.
â€¢ Time Machine. I don't know what I was expecting when I saw this demo'd on the the 2006 WWDC video, but I guess they didn't make it explicit that you would need an external hard drive. I suppose it makes sense, but this pretty much cripples the feature for me. I have an external hard drive, but I just make manual backups via iMSafe (or however you capitalise it). I just don't connect to my external hard drive often enough for Time Machine to be useful to me.
â€¢ Mail. I was excited about notes and to-do items when I saw them demo'd on the WWDC 2006 video. On that video, Steve Jobs said something about how there was going to be an SDK for the to-dos so that they would be system-wide and developers would be able to implement them easily in their own apps. Well, I don't find To-Dos and Notes half as useful as I thought I would. I expected to be able to drag e-mails into the To-Do area and have them automatically become a to-do item, and then be able to add some notes to them. No such luck. Notes, e-mails and to-dos all seem very separate, as though they are three separate applications banged together without much thought about true integration. Very much tacked on. I have used To-Dos a couple of times to try them out, but I think I'll be using Jesse Grosjean's TaskPaper
instead from now on. Stationery is pretty nice, but I doubt I'll ever use the feature.
â€¢ Text System. This is the main disappointment to me. There are a lot of under-the-hood tweaks and bug fixes, but from a user perspective the text system hasn't really changed since Tiger. Tables are still a buggy mess, as are bullet points. There have been no improvements to RTF or DOC import or export, and, even worse, Apple have introduced new DOCX and Open Office import/export that has the same limitations as DOC etc (no support for images and so on). The only noticeable difference is the addition of grammar checking, and that's a feature I never use anyway. What about better styles support? A built-in page layout view? And so forth. I was part-way through developing Scrivener when I upgraded to Tiger, and I was suddenly able to add support for bullets, tables and headers and footers while printing. Grammar checking is all that has come "for free" with Leopard.
All in all, when I sit down and try to think about what is great about Leopard over Tiger, I can't think of much. There is nothing in itself that is an absolute killer. However, Leopard seems to be greater than the sum of its parts. As a user experience, it blows Tiger out of the water. It just feels
so much more polished, modern, fast, smart... From interaction with the bug reporting system, I also know there are literally thousands of minor bugs that have been squashed.
Hope that helps.