The first question you've got to ask is what types of games do you like?

Personally I'd go through the NinjaBee demos and see if any of them tickle your fansy (for me I think the best is Cloning Clyde). If your looking for multiplayer fun, the recently released Undertow deserves a look at it's demo. Lastly if your looking for a title to sink many many hours into, then Puzzle Quest may be what your looking for.

It's still early in the morning, so I'm sure I've forgotten many games, but that's one of the best things about XBLA, the demos.

I second Tomacco's suggestions, but also look into the demos for:

Puzzle Fighter HD

Street Trace:NYC

Roboblitz

Personally I am sitting on 4600 MS points right now : / as I am waiting on the release of a few games still. When Roboblitz gets the multiplayer update I will pick it up, but I already own it on Steam so rebuying a game is not tops on my priority list.

Well, if you can wait till December 4th, Marble Blast Ultra is a great game and the new 400MSP price cannot be beat.

For fast paced (and brutal) arcade action, consider Alien Hominid.

If you like puzzle games Lumines Live is great. (This is already down to 800MSP)

Rhythm games: Every Extend Extra Extreme (a great trance game) or Boom Boom Rocket

I like shooting stuff so the Mutant Storm games are fun for me.

Free game worth a shot at least: Aegis Wing

Oh, and be sure to try out Pac Man Championship Edition. I hate original Pac Man but C.E. had me *hooked* for a while.

P.S. I hate to say it but don't waste your time with Yaris. Yikes. It may be free, but really...yikes.