I used one in Jan. of this year when I first got this phone. (My first Android) The only reason I used it is because they were all over the Market. So I thought it was a necessity for Androids. Then as I became more familiar with this OS, and did some research on the subject, I decided to 86 the Task Killer. My battery life is great without it. I convinced my step father to get rid of his on the EVO, and he has never been happier. But he had it set on AGGRESSIVE (killing everything when the screen goes blank) which defeats the whole purpose of battery conservation. None the less, he is better off without it.
Now, I will say, IMO Task Killers can be useful for certain people who set them up exactly for their needs. I don't run games or apps that suck power if left running in the background, so for me NO Task Killer works best.