I've only owned an X-Box since December so I'm not up to speed on all the titles out there, but being a long time RPG fan I find the X-Box to be rather lacking in this category.
Fable came with my X-Box and overall I found it to be a pretty interesting title.
It's pretty fun to play. The storyline is okay and the idea of being good or evil is a nice touch.
That said the game ends way way way to quickly. I finished it over a weekend.
There are some nice extras such as marriage, sex, collecting rings (or keys) to get special items etc. However I didn't find that these really added anything to the game and only seem to be there to get players distracted from the fact that this is a really short game.
Worth a look if you can find it used or you can always rent it.