(Sunday, October 5, 2008)
If you're looking to tool-up your bicycle as a car replacement, the Topeak Dynapack DX is worth considering. It's a trunk bag with a fully-integrated seatpost rack, allowing you to easily attach/detach the bag using a simple clip. The bag itself is a semi-rigid foam shell, large enough to accomodate a helmet and some gear (minipumps, etc.)
The ease with which you can remove the bag is what makes this great for commuters and others who use bicycles for errands - it makes it easy to strip your bike of easily-stolen valuables. If you have a flashing taillight, it would make sense to try to attach it to the bag - that way it comes with you when you leave the bike.
Yes, there are larger trunk bags, and of course you can also get panniers and handlebar bags. But the convenience of this bicycle trunk gives you Vespa-like carrying capacity with minimum installation, maximum security, and heck it looks pretty decent too.
Topeak Dynapack DX, about $75 from Amazon