Holy shitsticks. If you're an XBLA user, and you love shoot-em-ups, go and buy/download Geometry Wars 2 NOW.
More particles! I love how many more particles are in this version and how they toned down the background "grid" a little bit. I'm not a huge fan of the "curved" UI but after a while it isn't noticeable.
In one word: trancetastic. Great beats, gets you in the zone, and keeps your foot tapping. My wife won't admit it, but I saw her tapping her foot when I was playing :)