From a webmaster-to-user point of view, wmv isn't bad. If you know what you are doing you can keep the filesize low and the quality decent enough (for galleries and things like that).
From a webmaster-to-webmaster porint of view, I prefer huge mpeg's over wmv's. Yes, I know most noob-affiliates prefer wmv's they can just chop into little pieces but real webmasters ofcourse know how to encode stuff themselves
From a user point of view, I HATE WMV. The quality doesn't even come close to for example Xvid (in an avi container).