I like the Source bladder, it's my hydration system of choice, but I've not used the packs.
As far as actual packs go, like Pete says, pretty much everything around right now is hydration system compatible, so you can simply choose a pack that suits you and add a bladder of your choice - for my money it would be a Source bladder with a Camelbak bit valve.
As far as particular models go, they all have pros and cons, as you'd expect. The TNF Megamouth is ime very decent and well thought out, back system doesn't absorb water - it's a trampolene-type thing - and it has enough compartments etc, though no belt pockets.
For non-bounce, lightweight use, have a look at the Haglofs Ace packs, probably the Ace L. The elasticated waist belt thing is genius and works really well plus the weight sits nice and low on your back, otherwise quite conventional. Plenty of other options too, check the OMM sacs for a traditional, lightweight, compressible mountain marathon-style pack.
For something a bit left field, I'm trying to get hold of one of the Wingnut packs from the States - they sit the load very low on your back albeit with a harness as well, so combine some of the advantages of a lumbar pack with a more conventional harness syste.
