We used the Fisher Price Ocean Wonders one (on the most vigorous swing setting and the "shhh...shhh..." noise.
Pre-DS I was very anti-swings, but he was a baby who Would Not Nap. At all. Or, at least, not for more than 20 minutes while being held (in fact my first and very anguished post on MN was about that when he was 7 weeks).
I got the swing and could half-swaddle DS, jiggle him to sleep, and put him in the swing where he would nap (never got to the point where he'd actually go to sleep in it, or indeed deign to sit in it at all when awake). It transformed my life -- pre-swing I know my mother and MIL were convinced I was developing PND, but I was just so TIRED. Within days of getting the swing people were saying to me "Wow, you look so much better!" (OK, probably not tactful, but true).
Healthy Sleep Habits, Happy Child says they are bad because the child doesn't sleep as deeply in them as in a cot. To which I said Pshaw given that without it my child didn't sleep at all, and any sleep had to be deeper than none. Harvey Karp (Happiest Baby on the Block, etc.) is very keen on them, though.