I'm torn between all Plex, Boxee and Front Row, actually. I use iTunes to manage all my music and movies, and pictures are in iPhoto. Because of this, it seems I'm basically stuck with using Front Row for my digital media. Boxee is better than Plex with photos (if I recall,) but both are pretty bad. I don't want to have to rename my movies so the information can be pulled from IMDB... that's why I've spent so much time using Lostify to get the metadata correctly entered.
For me, Plex and Boxee serve one purpose: streaming online content. I prefer Boxee's navigation (not a lot of menus to drill down through, and I find it easier to locate content I want to watch.) I think Plex looks nicer, and they have some really great apps for streamed content (Bill Moyers' Journal, Charlie Rose.) Boxee offers CBS, WB, and Discovery Channel podcasts. Also, when it comes to Hulu... Boxee allows you to jump to a letter, which is much easier than scrolling through everything. I may end up using Hulu desktop anyhow, though.
If Plex could tie into iTunes and iPhoto seamlessly, I'd probably be done with Front Row and Boxee both. And while I find Plex's apps/content more desirable, it's not enough to be completely through with Boxee, and definitely not enough to be done with Front Row.