I was going to pick up Crackdown anyway, and this beta is just the icing on the cake. I really enjoyed the demo, and although I was skeptical because of the few problems it had, this beta should make it a worthwhile purchase.
Plus, I'm a huge Halo fan. I played the first two games to death (and still do, from time to time), and I can't lie that I'm excited for the beta more than I am for Crackdown.