This last weekend I used my new cycling specific rucksack for the first time. It was great! It's not super big, but it's big enough to get enough stuff in for a typical weekender, and it's very flexible. One of the best features is the big 'helmet' pocket at the front. Quite why you'd want to put your helmet in it I don't know, but I stuffed my waterproof jacket into it and that worked fine since you don't have to then put that in the rucksack compartments. It also makes it very easy to get on and off which I did several times on Monday due to the changing weather.
Apart from the 'helmet' pocket, the bag has three big main pockets, and several little ones. There is a small one on the front which is perfect for keys, cards and change, spare batteries, headphones, etc.. and one on each side which will easily hold a mobile phone, ipod, or a slim camera. There is a tiny pocket on the belt strap which I didn't use, but could fit some coins in. Two of the big pockets have subpockets - two net ones with zips and some zipless pockets. The bag is very versatile because of the number of pockets, and I certainly didn't fill it as full as it could've been. Indeed, next time I will probably put in a few extra clothes in case of wet weather. I did travel light this time, but when you're carrying everything and mountain biking, that's what you want! And you don't want things sloshing around when you descend rocky paths. I even managed to get a few books in and my Asus EEE PC laptop :) which the bag protected very well. I didn't notice any strange movements even on very rocky terrain, due to the fit of the bag. There is the clever little whistle on the strap, and the shoulder straps are slim and wide making it comfortable.
The only thing I can't comment on yet is the performance of the bag in the rain, because it didn't rain this weekend! But so far I'm very pleased with it!
I bought mine from wiggle.
The North Face Crank 25 rucksack, packed up, after I got back home ...
... and here's all the stuff that was in it!