I have a ton of music in iTunes, some of it purchased through the store but most of it ripped from original CDs. (For purposes of measurement, "ton" is defined as 16k songs across 1.3k albums.)
Said music resides in iTunes, because I have two iPods and an iPhone. Said iTunes resides on my Media Center machine. When I first started building it, I debated running OS X with Front Row or Windows MCE. I ended up deciding on MCE because of MyMovies 2, which is a killer piece of software. After all, the primary purpose of the machine was to be a movie jukebox, and Front Row is nowhere near ready to do that.
What had been missing, up until now, is the ability to see my iTunes music inside of Media Center. I tried a bunch of "homebrew" solutions, none of which was ever exactly what I wanted. The important criteria was that I wanted to be able to keep my iTunes database sync'd up with Media Center so that I could use the 10' UI to pick music. Turns out mceTunes was pretty much exactly what I wanted.
It can sync all my iTunes music -- including the protected stuff from the store -- and play it back within Media Center. It has some quirks, but all in all, it's exactly what I wanted. It keeps things up to date without me needing to fuss with anything, and the perf inside of MCE is excellent.