Does a great job holding water - IF you take the time to properly do it up after filling. Always check it yourself if someone else has 'helped' by filling it for you.
This WILL turn into a biohazard if you do not properly clean and dry it IMMEDIATELY after use. Mildew will build up and you'll need washing up liquid to scrub out the bladder with a sponge and the hose with a brush on the end of a wire. I'd recommend one of those cleaning kits by the way, for times where you 'forget' to get round to emptying it after use and it sits for a few weeks festering. Don't listen to people who say you should soak it in Milton - scrub out the mung by hand!
The neoprene cover for this hose keeps the sun off it, but at the same time doesn't let you see the mildew build-up. Play it safe, give it a scrub every now and then, and whenever it smells a little suspect.
This particular model is NOT compatible with the filter that Camelbak sells that claims to clip in between the hose and the bladder, despite what they say. You'll get wet if you try it!