used to always ride my tb in one. he's very sensitive in the mouth and so he found it bit more gentle and flexible. Also got roller ball in the center.
However he always managed to chew them to pieces and after replacing them every couple of months and talking to equine dentist decided to switch to french link snaffle instead.
so ok not pro's and con's but thats my experience with them... don't know if that helps at all!
My horse will only go in happy mouth bits as she's very sensitive. I normally have a straight bar snaffle which she loves. However if I go and something faster I use a straight bar dutch gag which makes the inside of her lips go hard and puffy. So I ride her in a hackamore for a few days after using the stronger bit to let her recover.
So good points are that they're soft and good for horses who don't like metal.
Bad points can rub (I guess it's rubbing) the inside of the mouth but a bit of vaseline and they're usually fine.
Good for fussy mouthed horses, but I imagine that some horses might see them as a bit of a joke and just run straight through them...
I think if the horse likes them - they're great! If not... well, they're not so good! xD