I have a Mantis rucksack, there was a review in PGI a little while ago, I can get everything in it including my air tank, I do have a drop forward on it too, but I bought a little quick release for the drop forward from
NW Paintball this got me round the problem of it all fitting. the rucksack is about half the price of the JT one, and the only difference I can see is the straps on the JT one are more heavily padded. Don't have a web address for Mantis but I'm sure someone will help you out, if only as an alternative rucksack to look at.
To answer what you can put in it, an Angel AIR .68 (larger size will fit), a pair of goggles, 2 hoppers, 4 barrels, spare pot full of spares, air fill rig, couple squeegees, angel charger, 3 jerseys, elbow pads, 3 pairs of gloves, batteries, etc, oh and an Angel and it still isn't full, can't get your battle pack in or your knee pads but then unless you want a Planet bag you can't do that with a JT rucksack either.