The Flip4Mac plug-in is probably the best $10 I have ever spent on software.
The official WMP player is, frankly, useless--aside from being slow, ugly, and having TERRIBLE video quality (particularly scaling), it has a nasty tendency to stall for no reason at all, and it is almost impossible to skip around in a movie--it takes several seconds for the video to kick back in, if it ever does (half the time if the movie is over a minute it just stalls permanently).
I use VLC and QT Player on a regular basis, and MPlayer for stubborn files on occasion, but none of them consistently play most of the WMV files I get my hands on, and they frequently have glitches when they do.
The Flip4Mac plug-in, however, not only produces nice looking video, but you can scrub around in a file remarkably well with only minimal stuttering--better, I daresay, than even most AVI files. It also rarely stalls.
It won't play the absolute newest WMV files, there are a few files it seems to loose synch on (the video plays much more slowly than the audio), and it refuses to play a lot of streaming video (not sure what the difference between what it will and won't play is yet), but in all it works the large majority of the time and I am so glad to be able to avoid MS's worthless player that I'd pay for it again if I needed to.