Honestly I don't even notice my rated vest now. [and it's one of those cheap bulky stiff things]
I don't always jump in it because it does occasionally get in the way [it doesn't quite fit... I'm a tricky size/shape apparently, nothing -quite- fits] but I won't get on my breaker without it [see photo attached, her 4th ride]. And I won't ride XC without it no matter what. If I went to an event and realised when I was getting ready to run XC, I'd scratch, without hesitation. A vest won't do much against a rotational fall, but people have been killed from being thrown into fences, and that type of fall/injury, a vest is brilliant for.
One day I want an air vest because that way it wouldn't be as hot, as bulky, or get in the way at all... until I needed it. Plus you only have to replace the air canister, not the whole thing, if you come off [provided, of course, nothing punctures the vest itself]. But they cost too much.