I've never used Acorn, but I have Pixelmator and I've been a Graphic Converter user for nearly 20 years.
I keep trying Pixelmator, but really can't warm to it. I suppose it's because I haven't yet discovered how it works; nothing seems to work the way I expect it to, so in the end I keep reverting to Graphic Converter. But it does have layers.
The only thing that I would like in Graphic Converter which still doesn't seem to be there is layers. Apart from that, it does everything I want and more, it is stable and the latest version has apparently been completely rebuilt in Cocoa, so it is a fully-fledged OS-X app.
Of course, I also have iPhoto, and the current version gives you much more control than earlier ones did, so apart again from layers, for most basic work it will probably be quite adequate, and of course its library function is something that neither Pixelmator nor GC have.
Another low-cost ($30) basic, but very usable image editor is PhotoComplete (http://www.funkypixels.com/
).Very straightforward UI, non-destructive editing, you can go back and change earlier edits without having to roll back through history. No library function or one-click upload to Picasa/Flickr/.me/whatever, but I guess the real big disadvantage is that it hasn't been under development for a few years it seems. Perhaps worth a try to see if it works under Lion and if you get on with it.
More expensive but much more powerful is LightZone (http://www.lightcrafts.com/lightzone/
). A lot of people reject it out of hand as it's cross-platform and written in Java — I must admit to not understanding why people reject things which do their job well, are stable, and in my experience no slower than the opposition, just because they're written in java, but still ... — Comes in two versions; the one without a library function is now up to $99, the library version if it's still available will be double that, I guess. I have yet to install it under Lion, but I usually have it as my external editor for iPhoto for dealing with photos that need real sorting out.
Edit: You might like to look at this, which I've just seen, by the developers of LightZone. http://www.lightcrafts.com/aurora/