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.