On my new XP-laden laptop, I installed an older version of Gaim, which happened to be a bit buggy (.6x series iirc). Previously on Windows, I used Trillian, which I thought to be generally quite good. So suffering a few annoying Gaim bugs, I tried to switch back to Trillian, and was amazed at how terrible their interface was.
So I grabbed an updated version of Gaim, and re-installed it, and was amazed. I've used Gaim on Gnu/Linux and BSD for a few years now, but always found it a bit out of place on a Windows desktop. The recent builds, though, with the built-in Wimp theme, and wonderful/magical instaler fit into a XP desktop seemlessly, AND the user interface is really quite nice.
Unlike Trillian (or *gasp* MSN), the Gaim interface is not at all obtrusive. It's simple to configure, and errors are reported with a hint of sanity. When I was setting up Trillian, it wouldn't connect to MSN, and gave no useful message to lead me to a solution. In fact, I had to search for the f##k#ng conection-log, and was then presented with obscure numerical codes.
In general Trillian configuration is retarded, and starting/stopping an account is worse. And, the interface hasn't improved since I last used Trillian regularly, which is at least a year ago (who doesn't improve a desktop app within a year??). Gaim, OTOH has improved considerably, on nearly every axis. It's even good enough that I plan to set it up for a few non-technical people I know who suffer life using MSN-messanger.
Kudos to the Gaim crew, it's a great app!