I used the Da Kine one for ages (previous version). One word - awesome.
I'm pretty dense (yeah yeah), so I sink even in saltwater (without a wetsuit), so a bit of extra floatation was great. Spent a season sailing in Egypt with one, no issues with being able to unhook, being unwieldy, etc. Great bouyancy, great for learning to waterstart, great protection from back-slaps when learning to loop... all-round recommended. I'd still be using mine if it wasn't in Egypt with the ex!
It also probably prevented a couple of hitting-the-mast-with-my-ribs injuries / bruises...
Thinking about it, I'm about to start trying Shuv-its... I'd quite like one again because I'm certain I'm gonna wind up on the boom pretty hard at some point.
As Zed said, probably no good for waves though.
EDIT - acts like a normal waist harness, not restrictive for freestyle (IMO)
EDIT1 - Love Da Kine stuff too, built to be bomb-proof. Never had anything DK fall apart until it had taken far more abuse than it should have done. Harnesses, backpacks, travel bags, everything. Build quality is simply superb from my experience. And no, I don't work for 'em