Like singlespeedstu I have a decathelon bag, although I have a forclaz 30air, and it really suits me well. Its on the larger side, and perhaps slighty heavier than a camelback mule, but I really do like it! Its designed more for an all day hike, but the upshot is its incredibaly comfortable, the weight is nice and low around my hips, and it has huge foam pads to hold it off you back allowing lots of air to circulate, whilst providing great back protection in the old tuck and roll dismount. The straps and nicely padded, and the waist band has two side pockets that you can reach whilst riding, usefull for your phone, or in my case, my inhalour should I want to take a cheeky puff without stopping and looking like a demic. The bag has a huge 30 litre capacitly, but pull the load straps up and its as thin as anything! and nothing shuffles about inside. Even comes with a little zip out extra waterproof hood for epically wet and muddy rides when you want to keep your bag and back dry.
I know it wont suit everyone, and chances are youll discard it because its not specifically aimed at cycling, but if your ever in a decathelon shop, just try one on. Ive recently been looking at something smaller for quick local rides out, and ive tryed loads of camelbacks etc in the local Evens, and in comparison they feel awfull and cheap, with the weight far to high up for my liking. Still, each to their own. I decided on a small saddle pack to keep my repair kit in for the local evening blast, and whislt the bags really not a problem, it is nice to ride bareback! 😆