Google Talk has just been released, and I'm giving it a try. I primarily use AIM, because it is almost a must at eBay, and because my wife is a long-time AOL/AIM user. I've always hated how cluttered AIM is, and I can't tell you how many times I've had to go searching through menu after menu to find the sound-related preferences.
My initial impressions of Google Talk are favorable. I like the uncluttered UI, and the basic features are intuitive to me. I like the UI for searching for "friends" (Google Talk's name for buddies), and I imagine it could be quite useful when there are hundreds of friends in your list. I also like the snap-to UI for IM windows.
Initial reaction by the media is that Google Talk is underwhelming. I disagree -- we don't need another IM client that packs in hundreds of features that just add clutter and confusion. Producing a simple, easy to use IM client is, to me, quite impressive. I'll reserve final judgment until I've used it more heavily, but right now I'd say that Google has another winner.