I've never fully converted to iTunes and have to admit that I sometimes use Windows Media Player to play my music. The problem is, your playslists become a mess as you switch from one to another and vice versa. Keeping your library in synch manually is a time consuming process - that is unless you use MusicBridge which does an amazing job of keeping them both in perfect harmony at all times.
MusicBridge does not copy songs between iTunes and WMP and it does not let WMP play AAC/Mp4 files. Rather, it simply allows you to copy meta-data - the unique ID's assigned to each song - between the two. For MusicBridge to work both WMP and iTunes must be pointing at the same set of files in the same location on your hard drive and those files must be playable in both applications. But don't worry - MusicBridge does not copy or move any music files from one application to the other.
The only downside is that you have to have iTunes and WMP running at the same time on the same machine although if you're a regularly user of both, this is not big deal for a program that saves you a hell of a lot of work and confusion.