As mentioned in another post, I ended up buying a Vaude Bike Alpin 25+5 (currently has 25% OFF at
Edinburgh Bicycle).
It's only my first week commuting with it, but I'm very happy
The mesh back system really does work – no sweaty back compared to using a much smaller Hydrapak Big Sur with it's foam pads against my back, and there are plenty of pockets and compartments to sensibly organise my stuff.
I really like the zipped 'floor' in the main body of the bag which allows you to use either one big storage area, or – as I prefer – a smaller lower section (I've got my overshoes and rain jacket in there) and larger upper section (perfect for change of clothes, lunch etc.). There's a small lower external pocket that currently houses my tools and tubes with room to spare, and that still leaves a large upper external, map pocket (?) and top pocket so I've got choices where to stick my wallet, keys and phone! Oh and as suggested by the product name, the whole main section can be expanded by 5L if required.
If not out of budget, I'd suggest it's well worth a look.