This bag is "the answer" for short tours and commuting. I've been using this bag since June 2005 on my daily, all weather, 7 mile one way commute to work in London.
For commuting it’s a perfect way to avoid wearing a backpack and is big enough for serious touring where panniers will be too much. I’ve really put the bag through a lot and it’s bombproof. The bag looks great – it’s got a truly old school, 1950’s England look – and has weathered attractively.
This bag, with an optional SQR mount, mounts on the seatpost. I’ve ridden day after day (touring and commuting) in constant soaking rain and my experience is that it is entirely waterproof.
It has one large main compartment and two side pockets - all very straightforward.
It’s rated at 24litres (about the size of a large daypack) and is bigger than I need on my commute. The extra size helps me transport shirts to work without wrinkling.
It is big enough for credit card touring. You only need more capacity if you’ll be camping, touring in the winter or plan on touring for more than a few weeks. To date it has endured tours through France, Poland, Slovakia, England and is expecting more soon. For my touring set up I use this bag in conjunction with a frame pack for tools/spares and a handlebar bag for valuables and maps.
The SQR is a brilliant feature. It allows me to attached and remove the bag from the bike in a matter of seconds. Once mounted, the bag moves around some and can swing back and forth when climbing out of the saddle. This has not caused me any problems. I’ve ridden offroad, on cobblestone and on terrible Eastern Bloc country roads with the bag and it’s absolutely, totally secure. I've had a few accidents and occassionally dropped the bike - the mount system has always survived intact - it's not made of china. A loaded seatpost bag obviously raises the bikes centre of gravity and this may take some getting used to – particularly for those more familiar with panniers which lower the centre of gravity. I feel less secure descending at more than 40mph with this higher centre of gravity than with panniers or my BOB but, on a loaded bike, it's probably ok to go a bit slower.
Here's the bag in Poland/Slovakia (first picture on Donohue):
Here's the bag in Brittany/Normandy (first picture):