I've seen one guy use them every session, it has some merit I guess if you want to stay out.

I had a bump n jump impact vest years ago with pack built in. Was great actually, pull up and have a few minutes rest and a drink, then take off again.

Biggest problem is that they are hard to keep clean. The packs just don't dry inside, so they grow mould or even if you can't see anything they can be slimy inside. There's a special drying hangar thing you can get and tablets, but that was all too much hard work for me.

I've used a run of the mill classic style camelbak 1.5l hydration pack in the past for longer 3-4hr sessions and worked very well.

In saying that I've only really used it for long downwinders so didn't spend much time in the water. I found that when i was in the water it would ride up. It would be a pain if you were boosting or spending more time falling off.

It was something small like:

http://shop.camelbak.com/classic/d/50012_c_1120_cl_2520

Ive used a camelbak a few times, I have one of the smaller ones so it doesnt move around much and you dont notice it whilst kiting.

RE mould / keeping clean, you can either keep the bladder in the freezer (empty) or clean it out with PSR which is commonly used to clean bear bottles for home brew

I also used one when learning, found that it was great to have fresh water to keep me hydrated whilst expelling lots of energy required in the learning process wrestling the kite and board etc. small camelback style. Did me well, but have grown out of the idea through progression.