I'll chime in on the OTRmost Hydration Pack. I've been riding with it for about a year now with some, um, less than graceful get offs. The pack took the brunt and held up great. The only damage other than getting dirty was a little cut on the mesh where my pump lives. The 1.75 liter bladder holds enough water for a two hour ride on a warm day and the pockets in the belt are handy for quick access to snacks, sunscreen, or other small items.
The lower pocket is big enough for tools, a spare tube, and a small light weight wind/rain/puffy jacket.
The expandable top pocket is plenty big for a first aid kit, a larger jacket or wind pants, and plenty of other stuff. I rarely have much in it but it was handy for carrying spare batteries for a hedge trimmer while doing trail maintenance.
I like the way the bladder has flaps instead of a pocket. It easy to remove the bladder to fill or clean.
Other nice features are the raised padding on the back that provides at least a little ventilation, the magnetic catch for the water tube (super handy and very functional), and ventilated shoulder straps.
I also own the Kulkea Micro Pack and Thermal Trekker and am just as pleased with this as the others. Kulkea's attention to detail and use of sturdy materials is worth the price of admission.