Update: May 2013: read here for an updated, in-depth review of this hub, and a warning about poor reliability of the coaster brake version of this hub. I use only freewheeling internally geared hubs, never coaster brake versions, as coaster brakes add unwanted stress & heat to the internals. But this particular hub seems particularly fragile in that regard. I own and use 2 of these hubs and highly recommend them. But both are freewheeling versions!
The S2 is an internally geared hub with 2 speeds. You shift gears by pedaling backward 1/4 to 1/2 turn. It requires no cables or shifters. The low gear is 1:1 and the high gear is 1.38:1. It is available both with, and without, a coaster brake. I opted for the latter.
I recently installed on on my daughter's BMX bike. A colleague at work loaned one to me, she liked it. So I purchased the hub on eBay , computed the right spoke lengths with the Edd Spoke Length Calculator , double checked it with the Bike School Spoke Length Calculator , bought the spokes at Bike Parts USA, and built the wheel with the new hub.
Here is Emma with her customized S2 Duomatic BMX bike:
Total cost was just under $100 because I built the wheel myself and reused the rim:
Her bike's rim was generic aluminum with no brand name or specs. So to get the ERD (effective rim diameter) for the spoke calculator, I wrapped a string around the rim next to the spoke nipples. Measured the length, divided by PI. I found this easier and more accurate than conventional ERD measuring techniques.
(Updated May 2013)
Shifting is 95+% reliable. It rarely takes a second try, but when it does the backpedal is so quick it's no big deal. The shift (whether up or down) is a very quick 1/8 turn backpedal. If you spin the pedals backward it shifts back and forth repeatedly.
There is no obvious way to know what gear it's in. It is a bit louder in high gear, both when coasting/freewheeling and when pedaling. Low gear is dead silent when pedaling, high gear has a light clicking sound.
The difference from low to high gear is 38%. This is roughly equal to 2-3 gears on a normal 10-18 speed bike.
My daughter's bike was geared 34-16 with a 20" wheel, making about 48 gear inches (including the tire). Since the S2's low gear is 1:1, and she needed a lower low gear for the hills around here, I needed a shorter ratio - bigger rear sprocket or smaller front. The front is a pain to swap, so I put on a bigger rear. This required a longer chain. I had a spare chain and links so that was a snap. I opted for 34-20, about 80% of the original gear ratio. That makes for 38 gear inches in low and 53 in high. So her low gear is 20% lower and high is 10% higher. This turned out to be perfect. I have a 19 rear sprocket to swap in when she gets a bit stronger.
(Updated May 2013)
About a year later, Emma's gotten bigger and stronger, so I regeared her bike so she can ride faster and keep up with us better. I swapped the rear 20 with a 17, making it about 15% taller. That makes for about 44 gear-inches in low and about 60 in high. Compared to the bike's original single speed gearing, the new low is about 6% lower, high is 30% higher. Emma likes the new gearing, as it helps her keep up with us better. It really did make her faster.
I used to run while she rode her bike, and I had to give her pushes up hills. But now I can't keep up with her anymore when running, so we ride bikes together. This is a lot more fun. She can go up the hills on her own, where before she complained and needed pushes. On the flats, she can cruise at around 12 mph which is faster than she could before. It made a big improvement.
Overall this is a nice piece of bike equipment. It's simple & effective and needs no cables or shifters. Most people buying it are going to have to pay someone to build a custom wheel, so it can be expensive - expect close to $200. But if you can build your own wheel, or get a friend to do it for you, the price isn't too bad.