I recently renewed. I'm a member May 2013. I have no idea what my membership number is. For me the citibike offers a very viable alternative to the subway for a leg of my daily commute. Given the cost of a monthly unlimited metrocard vs. the annual citibike membership, the citibike membership pays for itself provided I do my part and get out there and ride.
I'm @ 900 trips which they estimate translates to @ 2,200 miles.
Overall I'm very satisfied with the service. My one complaint is that they eliminated the bike valet on 31st between 8th and 9th and they moved the station 29th and 9th. I tend to get to Penn Station late enough that finding a dock is a real challenge. The dock at 29th and 9th got moved 27th and 9th. As a result I usually end up using the dock at 26th and 8th and walking a few block to Penn. Too many times I found myself looping back from 33rd and 8th all the way back down 9th to 27th to find a dock.