So I made a thread here a few days ago asking about backpacks, and I ended up buying the Deuter Race EXP Air.
It feels very well made and can fit quite a bit of stuff and it has an expansion facility so that you can carry even more. Without using the expansion facility I managed to cram in:
- 500ml bottle of water
- cable lock
- tube, tyre levers and a multi-tool
- sunglasses case
with space for probably another few t-shirts. It has a built in raincover which can use up a bit of space inside the bag - I will use this all the time so I have more space in the bag and also it is yellow so people can see me! I don't know how it works in the rain though but if it is poor then it is easily lined with a plastic sheet.
The "Air" feature is quite good, you still get a bit of a sweaty back but it isn't as bad as without it. The straps support the bag pretty well - chest and waist straps, and the shoulder straps are very comfortable and aren't sweaty. The waist straps are a bit thin and could have a bit more support but this can be added if necessary with a little modification.
I plan to get a hydration pack for it and it has a space for this inside the bag and a clip on the shoulder strap. The bag can hit your helmet a bit, but if you loosen the shoulder straps this is fixed and the bag still feels supported.
Overall a good bag, and I gained 1mph over my panniers!