I managed to finally, after some wrangling around, get a copy of Guitar Hero 3 for PC. What do I have to report? For one, the XBox 360 controller works perfectly fine with the game - yes, this includes the new GH3 wireless one. Problems? I've finally come to see what all the reviews that complain talk about. The lag. Not really so much the lag, more the unstable frame rate. The game doesn't actually lag, but the frame rate is unstable. Why? Because the camera moves around so much. When you are at a spot with little bling-bling to render, it plays real smooth (exactly like the console versions). When the camera zooms out, then suddenly the frame rate lowers a little - and your notes start going slightly out of sync.
If the only version you've played is the PC version, then there's a very high likelihood you won't notice. On my dated computer, it took me a while to actually pinpoint the problem. The frame rate is unstable - and the video lag calibration doesn't help. When I do a video lag calibration on the PC, I get 40-70ms, of which the correct correction is zero - which points out to me, the frame rate is somehow unstable - even on the calibration screen. When I do a similar calibration on the XBox 360, it will always return me 0ms. (I use a computer monitor as my HD display, so no surprise there.)
And if you've played the console version, you'll find yourself missing some strange notes at different times in random places. That being said, once you get used to it, it's not really a problem - it still is Guitar Hero. Of course, if you plan on hitting all of those hammer-on sequences in Through the Fire and Flames, then it's probably time to get a new computer.
The only way I can think of that Aspyr can fix it, is to add an option to fix the viewpoint. Sure, it'll be boring, but it'll get rid of the annoying unstable frame rate that can really screw with your eyes.
Since I have no way to be sure whether or not it runs smoothly on a newer far more powerful computer, I'd have to recommend against buying it, and sticking to one of the numerous console versions. If you don't have a console, then chances are, you shouldn't be playing Guitar Hero anyway.