I’ve got a Camelback Lobo with a 2ltr bladder, though the Lobo can take 3ltr bladder. The Lobo is perfect for a day out at a trail centre, or if you’re out for a few hrs. Just enough room for a couple of inner tubes, decent multi tool, a couple of CO2 canisters, a spare pair of brake pads, a spare mech hangar, a few gel packs, a packet of sweets, keys, iPhone (or other smartphone, or non-smartphone). It has an expandable pocket for a rolled up kagool and a decent sized mini pump. If I were out on an all-day epic out in the sticks and wanted to carry a few more spares and kit to be even more self sufficient, you’d need a larger pack. The Camelback bladder system really is very good. Not tried the other brands, I’m sure they’re just as good, but the Camelbacks are brilliant bits of kit.
I just wish they, or some other brand out ther would offer one that has all the kit in the middle and have two 1 or 1.5 ltr bladders either side with a straw over each shoulder. I find that having the bladder between your back and your kit is uncomfortable and having two bladders would be useful if you wanted to have water in one and something else in the other, like High 5 or beer!