Enclave rocks....it's very challenging, but it's awesome. Same developer as Chronicles of Riddick for Xbox BTW.
I would recommend the PC version of Enclave over the Xbox version though; it controls better and got a few small tweaks to make the game a little less frustrating at times. Get it either way.
Deathrow's awesome too, although I can't really recommend Buffy anymore. Felt too choppy last time I tried playing it...
I'd love to see a Deathrow sequel with Live play enabled. And all the teams rock...you get better players as you go through the main mode anyway. I love using the Seacats because of their Russian accents and lines like "I hope you've picked out a dress to wear for after I've CRUSHED YOUR BALLS in." Gotta love the cussing in Deathrow.