I use this pack for Mountain Biking and find it ideal. It has a properly ventilated back which makes a huge difference in keeping you cool and keeping your hydration pack cool as well.
Personally I think the 15 is the ideal volume; its light but still has enough room for all your bits. One of the compartments has pockets for all your tools, pumps etc. There is a wee compartment for phones and wallet and the main compartment for hydration and any bigger bits and pieces like lunch, tubes etc.
My only very small gripe is that the piping running around the top of the tool compartment gets slightly in the way of your zip, making it a little juttery to open. No big shakes, each zip has a nice large rubbery finger loop so it wouldn't spoil anyones day