I played UT2K4 once and found it a little boring.
CS:S is great fun though, and BF2 can be if you get a good game with good people and don't get spawncamped and baseraped all the time.
America's Army was alright, but got really old and it's not really that good. I've not tried v2.4 yet though.
Master of common sense. If you don't like it, stop reading.