Popular Science selected the infinitely geared bike as the "Best of What's New" in recreation for 2007. It's called The Ride by Ellsworth Bikes. Instead of 21 discrete gears, this bike has ball bearings that control two discs, at different angles to smoothly move through a range of gear ratios. You control the gears by twisting the handlebar. It's an idea based on a 1450 Leonardo Da Vinci sketch, so they call the gear system NuVinci. See it in action in this explanatory video. A fact sheet is available in .pdf form.
OK, I'm impressed. But to get a better view, I asked a serious cyclist.
These things are perfect for the casual cyclist. They are sealed up, so they don't get dirty no matter what the weather. They don't need to be adjusted and are virtually maintenance free. You don't need to worry about gear ratios and which chainrings to use in what combination. You can just get on and pedal. That's why this is really ideal for "casual or commuter" cyclists. And as I said, internal hubs have special advantages for recumbents and trikes.
He went on to explain that for a serious cyclists, there are a couple of drawbacks. The gearshift assembly weighs nine pounds by itself, adding a lot of weight to a bike that would be carried a lot. Also, hardcore cyclists like to change out freewheels and gears, which is not feasible with the NuVinci system. But even these cyclists are impressed with the technology. Future advances in the NuVinci system may allow for toters and tinkerers, and the price of The Ride (or other infinite gear models) may eventually come down from the current $2,995.00.
Many thanks to PAgent for assistance in this article.