I have the H.A.W.G.. It's the best hydration pack I've seen. It's flow through back panel is superb. Very comfortable and cool. The pack balances nicely on your hips when the hipbelt is tightened. The bungee cord at the bottom allows me to bring a blanket on trips, freeing up space in my pack (I actually use one of those Coleman ponchos with the fleece lining - perfect for a picnic).
You may not need as much space as you think. I am able to fit a 12 can Igloo cooler inside the main compartment, converting the whole interior to a cooler. I forgot the name of this cooler, but it's virtually uninsulated, and rolls up like a dry bag. A cooler inside the pack doesn't really need to be insulated, since stuff stays quite cool. This is great for picnics also, or for when you just want to bring a fair amount of food. My alcohol stove still fits inside the main compartment on top of the cooler, as long as it's not completely full.
In the smaller compartment, I am able to fit utensils, condiments, etc., a flashlight, a headlamp, and a small towel, and there's still more room. The outer mesh compartment holds a 16oz Lexan Nalgene bottle - excellent for coffee, or to drink water out of. I don't like to drink directly out of the hose. I prefer to take big gulps, and don't like water as cold as ice.
It's also possible to wrap the upper compression strap around your car passenger seat headrest, so that the pack is held against the passenger seat, and won't move if you slam on the breaks. So it adapts to a car nicely, too.
I am very happy with mine, and can't recommend it enough.