I've got a pretty decent collection of backwards compatible games (probably 30+ like KOTOR I&II, Halo, GTA, Splinter Cell, etc.), most of which I bought used for "cheap" (i.e. under $10) at the time. I don't play them at all on the Xbox 360. Occassionally I'll bust out a game of NHL Hitz on the old Xbox though.
I still can't convince myself to part with them because I keep thinking I'm going to get around to playing them, and I'm not willing to sell them for $3 a piece (or whatever the going rate). As long as I don't need the space, its no biggie, but at some point I'll need to dump them with the Xbox and get what I can for them.