I've had one for about a month now and use it for about 15 - 20 mins each day. The co-ordination in my left hand has greatly improved, it has also strengthened the tendons in my wrists (left wrist a significant difference) and it has enlarged and toned the muscles in my forearms. I bought one to improve my grip which should help with my riding and other activities i do like windsurfing, judo, fixing my car as well as general day to day activities.
I got the neon blue one from play.com for around £16 - got 12,something before but accidentaly reset the counter 11,947 is my best so far.
Basically they're amazing, buy one, end of!