From looking around a bit, I see that it was announced in the spring, but is just getting out to bike shops in the past few weeks. I know that it wasn't at Harris Cyclery about 6 weeks ago when I was looking at cassette options, now it is.
Apparently this was intended for 29er MTB's, but with the right Shimano long-cage rear derailleurs could have touring applications as well. If you already have a triple with a small (22, 24 or 26 tooth) chainring, this may not matter, but it could be useful if:
- You have a bike with a compact 50/34 double crankset. Putting a 12/36 cassette on the back gives you a low gear of around 25 gear inches.
- You have a bike with a "road" triple crank with a 30-tooth inner chainring. Adding this cassette to the rear gives you a low gear around 22 gear inches.
Harris states: "This cassette will index with any Shimano-type 9-speed shifter, but does require a Shimano rear derailer with Shadow technology."