Are Personal Flotation Devices (PFDs/life vests/life jackets) good for rafting? It may seem like a silly question, but I have heard several times now that they increase your danger due to the risk of entanglement.
No question about it -- use a life vest. Make sure you're using a secure, tight-fitting jacket, though. The ones I've always used have several straps across the front so you can get a good, snug fit. If the jacket isn't fitted properly, you stand a good chance of the jacket popping up around your face when you go in the drink. A tight-fitting jacket also helps the people in the raft haul your sogginess back into the boat -- it's a lot safer and more reliable to grab your jacket to haul you in than to try to pull your arms out of their sockets.
While you're at it, don't forget a lid. When you get sucked under the boat, you may go head-first into the rocks, and you'll be glad you're wearing a helmet.
The instructions I've always heard are unequivocal - always wear a life vest. By all means make sure it fits well and lose straps are tucked away to minimise entanglement, but, when in a sport where there is real risk of falling in water and being knocked unconscious definitely wear a life vest.
No question, life vests are a net reduction in risk.
It's trivially easy to check yourself and your gear over and eliminate any potential risk of entanglement beforehand.
It's not so easy to get control of your body once you've been unexpectedly tossed into the drink.