Long time MP user, switched to XBMC when Camelot came out - couldnt be happier!
1. I find the metadata scanning works better than Moving Pictures plugin on MP - tends to find movies more accurately (I got sick of continually entering data manually)
2. I could never get the auto refresh rate changer in MP to work with my Samsung 1080 32" tv. No matter what settings I tried, the output caused judder. I found the refresh rate changer in XBMC worked perfectly - Im VERY happy with this aspect in particular, after spending literally a year trying to get MP doing refresh correctly.
These days I use MP for all my TV recording (love the 12-line guide and the Comskip feature) and the OnlineVideos plugin, which I really like, but I use XBMC for all my 'channel b1t-t0rrent' viewing. (the forum actually asterisks it out if you write the full word of B__T______ in letters!!)
I tried MP RC a few days ago, but still have the refresh rate issue with it. Will keep an eye out for 1.1 final and have another go when its released.
For now,though, Im very happy with XBMC/MP combo.