XBMC is by far and away the best htpc front end. I used to use Mediabrowser and it doesn't compare in any way. I almost bought a new computer after how slow Mediabrowser was, then I tried xbmc and realized my computer wasn't slow, it was Mediabrowser.
What makes xbmc the best:
-FAST (it look Mediabrowser 8-10 seconds just to load all my files and it had to do this EVERYTIME it was restarted. xbmc only takes as long as it takes the program to open and boot which is about 1.5 seconds)
-Plays everything without codecs
-Plays packed rars
-Every little feature is customizable
-Content scraper is more robust. Mediabrowser chokes when trying to scrape tv shows with dates instead of season/episode number. It also won't scrape shows that are in a folder inside of another folder, or has no season folder at all.
-Auto refresh change for 24p content
-Amazing plugins! (SportsDevil, ESPN, Hulu, MTV, G4TV, Navi-X, Free Cable, Revision 3, Icefilms, just to name a few)
-Did I mention it was lightening fast?
I still have to use WMC for dvr, but for everything else it's xbmc.