OK, I admit it. I'm a big, big fan of dynamo lighting on a bike. Why? Well, there's a number of reasons.
First, dynamo lighting is always available. Whether it's a bottle dynamo, a bottom bracket model, or a hub dynamo, it's always attached to your bike, and it never needs charging. There's no need to remember to attach it when you leave, nor a need to remove it when you park the bike for that grocery trip, or when it's parked at work. When you need it, you switch on the headlight, and, if you have a tail light, too, they both come on and supply you with the light you need. In fact, the technology on dynamo lighting has evolved to the point that you can purchase a wide variety of lights with the ability to sense when its dark and turn on the headlight (and taillight) automatically! What could be more convenient than always having light, exactly when you need it?
Second, dynamo lighting is generally quite high in quality and reliability. Dynamo lighting is very popular in Europe and two of the leading dynamo light suppliers are based in Germany - Schmidt and Busch And Mueller (B&M). There are some very stringent safety standards and regulations for lights and bikes in the EU, and as a result, we get a product that is very high quality across the board. Whether it's a halogen bulb light designed for economy, or an LED light that you won't need to replace often, if at all, the light patterns have been refined and the products from hub to headlight are all designed with reliability and functionality in mind. These are headlights many Europeans use because they have bikes and not cars for transportation, so they have to be top notch.
Third, a dynamo lighting system can also add versatility/utility to a touring or utility bike. One of the newer products available is a set up that allows you to charge/use your mobile phone/GPS or other USB powered device while on the bike. While this may not be a big issue for commuters, it does allow a cyclist on tour to keep his electronics fully charged as he rides to the next destination, rather than needing to limit its use to ensure adequate battery life.
Finally, and it's an important point for me: Dynamo light is, relatively speaking, good for the environment! Forget about batteries that not only are environmentally unfriendly to manufacture, but also go straight to the landfill when they are discharged. Even if they're rechargeable, the batteries still have many toxins and exotic metals used to create them and allow them to hold their charge, and if they are rechargeable, odds are that the power to charge them comes from some non-green power generation. Dynamos, by contrast, might have similiar, or slightly reduced environmental impact from manufacturing, but are powered by you.
With all these advantages, it may seem that dynamo lighting is the perfect solution for a bike. Mostly, that's true, but there are two primary considerations before getting a dynamo light.
The first issue with dynamo lighting is cost. A dynamo system will never be as cheap as a small battery powered light, but it's a far superior setup. While you can invest $400-500 in building up the lightest Schmidt dynamo hub into a top notch wheel and then getting some high quality, top of the line lights, you can also get something nearly as good and every bit as functional for far less. For example, Sanyo offers a reliable, reasonably high quality dynamo front hub for $40. You can get it from some places (including Green Path Cycles, soon) as a prebuilt wheel, too, for anywhere from $75 and up, so cost-wise, it doesn't have to be a large jump from a traditional front wheel. Dynamo hubs are currently available from Sanyo, Shimano, SRAM, Sturmey Archer, and Schmidt, and are generally not too much more compared to a regular front hub.
For those who don't want to spring for a whole wheel, there's also the option of running a bottle generator, that runs off the tire sidewall, or a bottom bracket generator, which runs off the smooth center tread of your rear wheel. Both of these choices, while not as weatherproof or convenient as a hub, are good economical choices for powering some dynamo lights.
Likewise, in addition to affordable hub options, the lights themselves cover a wide range of costs. A simple halogen dynamo powered light can be had for $30 or so, and an entry level LED light for around $40-50. You can certainly spend more, just like you can spend more to get a battery powered stadium light to use on your mountain bike, but you don't need to for many applications. A taillight, too, is affordable, usually running about $20-30, again comparable to its battery operated counterpart.
The second issue with the dynamo setup is weight. Really, a dynamo setup doesn't necessarily weigh much more than a traditional battery light, although some of the newer state of the art LED setups have very light batteries, but rather that the weight is always with you. A dynamo hub can add a pound to the bike, which can sometimes be concerning to the weight weenies out there worried about every pound. But, when you consider that having other lights is also added weight, and that the added weight, as a percentage of your total weight - e.g. body weight, clothes/gear weight, bike weight - the added weight of the dynamo system is fairly minimal, especially considering the safety benefits it supplies. True, it's not for a race bike, but for a utility bike, a commutter, or a touring bike, dynamo lighting is a small amount of weight for something you'd need to have anyway, at least part of the time.
So, that's why I love dynamo lights, and also why Green Path Cycles is looking to become a leader in supplying dynamo lighting systems for all kinds of cyclists. It's better and it's greener. What's not to love?