Originally Posted by PaulRivers

But there are some drawbacks to dynamo wheels and light -
1. There's still *some* drag. It's really not a lot. I cannot tell while riding whether the dynamo is on or off, and with the dynamo off my wheel still spins a rather long time if I spin it by hand and just let it spin to a rest. But on really long trips (did 70 miles in 2 days) I got this feeling like turning it off made things a tiny bit smoother somehow.
2. As you can see from what I mentioned, they can be expensive (since you usually already have a wheel you're replacing). A really nice and lightweight one costs $300. You can pay $400 for the top end SON hub built into a lightweight wheel, or $150 at the lower end (like I just did). And you haven't bought a light yet, which costs between $100 and $200 (the Cyo, at $100, is nearly as good as the $200 ones though, to be fair, so say $100 for the front light, $25 for the rear, and another $25 in wiring - but that's assuming you install it yourself).
3. An Ultegra hub is 200 grams. The lightest Shimano dynohub is supposedly 490 grams. I'm not 100% sure, but they also may not come in anything less than a 32 spoke version. So it will add a certain amount of weight to your front wheel.
4. IMO, dynamo lights put out enough light to bike by, but not necessarily enough light to completely comfortably bike by. For example, my Cyo tends to put out enough light on the road, but not much to the sides - city riding where you can kinda see everything is great, as is biking in the moonlight. Biking through the woods where I feel like something might jump out at me means I bring my wider beam battery light as well. To be fair though, none of my battery lights light up the road as nicely and evenly as the Cyo.