Speaking from experience and being in the '
AppHub*' community for years, you will not see
many sales (maybe, at best, about 10 people earn a living, 1-3 with 6 digits a year... those guys are an
extreme rarity, you will likely get about the
low-mid 3-4 digits in a
year from a single project, people tend to get the money for their submission back but not much else) but getting the game on XBLIG is a great learning experience and easy on beginners to submission processes. Also the joy of seeing YOUR game running on a home console and on the digital distribution service is immense.
In short: Pros: Joyous, simple, fun Cons: Not monetarily worth the effort for most games made.
* To me AppHub is still the Creator's Club, and Kinect is still Natal.