The player functionality makes .WMV into a more friendly format, allowing me to actually scrub through movies (what a concept -- are you listening, MicroSoft?) and use all the QuickTime Pro features I expected. There is very noticeable computation when accessing Windows Media as QT media, but it does not seem inordinate. My only complaint is that it seems to break the Windows Media plugin, and embedded .WMV files no longer work in Safari and QuickTime Player is not launched as the Preference Pane would indicate!
The import/export functionality seems to work just as expected. I was making DVDs out of .WMV files I had laying around in minutes. I can write to .WMV without problems. There is some stuttering in authoring apps when working with WM content, but I will cut Flip4Mac a break -- I don't know whether it is their fault or not. Perhaps WM just stinks; MS could not even make a player that scrubs. Regardless of the cause, I am not disappointed.
Lastly, I will say that I am disappointed in the price, but I am not a design professional -- just a hobbyist. If you are a pro and you work with Windows Media, it seems like this is a real winner.