This rucksack is great when wearing any form of high visibility clothing as it does not create a "void" on the back like a normal rucksack would thus reducing your visibility.
It is a good size for a day sack and, although it may not last long if abused with really heavy weights (i.e. packed with loads of books), I carry my cycle repair kit, rain gear, a "couple of books", lunch and other stuff such as my lights when off the bike with no problem. This also leaves my pannier bags for heavier, bulky stuff.
The expanding section for a cycle helmet is clever as you still have only one bag when you get off the bike.
I was also impressed with the inclusion of a integrated rubber grommet on the top of the bag that allows you to pass a headphone jack into the bag and listen to your MP3 player held safe in a small pocket just inside the bag opening. Keeps your gadget safe, discrete and protected from rain.
Only suggestion is the that inclusion of a chest strap for total stability would have made this a perfect bag.
I would happily recommend this to anyone.
Happy cycling ;-)