Check out the BBC show Bang Goes the Theory. They did a special a few months ago called the Human Power Station, where they hooked up a room full of stationary bikes to generators and used it to power a model house and had a family that didn't know what was going on live in it for a day. It took like 50 cyclists sweating like mad to power the peak load, and 4 or 5 even just to power the clocks and such when the family went out for a walk. And that was fairly fit cyclists with ALL of their power going into high efficiency generators.

At a wild guess, with bottle generators, I'd say that the BEST you could probably do after all the inefficiencies would be to light a high efficiency light source (LED or maybe fluorescent) for about 3 to 5 minutes per hour of cycling, unless you really put enough generator onto it to cause your effort to go up a lot.

In the end, I'd guess that over the course of a year's commuting, you might be able to harvest 50 to 80 cents worth of electricity, MAYBE. It might be a fun project, but honestly, if you're going to put ANY money into this at all, spend $30 on a solar panel and use that instead. A cheap solar panel will put out 5 or 10 watts, but it'll do it continuously all day.