I bought this for longer runs since I don't like carrying bottles or leaving them lying around the route. I've used it twice now. Filling is fine once you get used to it. Once its filled you can invert it then suck out the excess air - that stops the liquid from sploshing around. It fits tightly and doesn't wobble about or rub even after a couple of hours out. As it's on the back you don't notice the weight (although so far I've only filled it to 2litres not the max 3) Given how well it fits I think it'll be fine for biking as well. I bought the cleaning kit as well and keep the bladder in the freezer between runs, which will hopefully mean no mould build-up...
In summary - perfect so far; I'll try to remember to update this review once I've had it for a while.