I've used them since the early 90s - never had a problem with them. Their real advantage is offroad - as they stay out of the muck spray from the front wheel. That said, I quite like to use it on-road too, as I can carry more liquid than a bottle, plus its handy for storing a waterproof, tools & food. Having the tube close to your mouth is surely safer than having to reach for a bottle.
The newer ones have a very large filling port, and all you need to do to keep them clean is rinse with clean water and store the bladder in the freezer. I suppose your back does get a bit sweaty, but hey, it's a bike ride.