It has been about a month since I started using TopDesk as an alternative to the default Windows task switcher and it has been a wonderful experience. There have been a few hiccups because software, well, is software, but being able to mimic the Vista Flip3D and Mac's Exposé effect is really cool. Most of the problems I've encountered are known issues, but I think for the sake of disclosure, they ought to be discussed.
The biggest issue I have is that of copy and paste in Firefox. Supposedly this is a bug in Firefox itself, but it wasn't made manifest until I installed TopDesk. Every once in a while Firefox will not do a copy/cut function inside the browser. Since my life pretty much revolves being able to copy words/URLs/etc to and from Firefox, this gets annoying really fast. Top Desk's answer: get Mozilla to fix their browser. I'm not sure what TopDesk is doing that causes Firefox to malfunction, but it makes for a bad experience nonetheless. It would be nice if TopDesk were fixed so that Firefox doesn't break, either.
Another problem is the way TopDesk behaves when playing a full screen game, such as Half-Life 2. TopDesk seems to cause what I term 'keyboard lag', where there is a painful delay from when you press a key and when the action actually occurs on screen. The remedy, of course, is to disable TopDesk when playing games, but sometimes I don't remember until it is too late. This bug is being addressed in the next release (date unknown yet)(Otaku software has released v1.5), but is painful until it actually comes out.
The last problem I have with TopDesk is it's inability to manipulate child windows. The biggest example of this is using the 'Hide' function where all windows are lowered off screen. This effect is similar to using the 'Windows Key+D' combination that is native to Windows that minimizes all windows. Unfortunately TopDesk can't hide child windows of Photoshop. If I am editing several photos and they aren't maximized within Photoshop, every window gets lowered except the photos I'm working on. In essence it defeats the purpose of hiding the windows. The other problem with child windows (from Photoshop, again) is that unless they are maximized, TopDesk will not draw them when you do the task switching effect. This one is more a gripe than anything, but why set expectations low?
Despite its problem TopDesk is slick piece of software. For not being apart of the original Windows API, TopDesk certainly does an excellent job of increasing productivity and making Windows XP a 'joy to use'. For $15 I don't think you can complain too much--except for the copy/paste issue. And, to be perfectly honest, I haven't had this much fun since I installed ObejctDock. So, ante up the $15 and get yourself a pretty descent piece of software.