You say you have Shimano Pedals/Cleats. How about Shimano cranks? If you don't mind being wedded to Shimano cranks, consider Pioneer Powermeters

I have more than one and they have been rock solid and as accurate as the Powertap wheels that I switched from. I love them. In particular, I liked the fact that I could buy the kit and have them installed on existing Shimano cranks; I didn't need to buy new cranks just to get them.

However I loved Shimano cranks before so it was not a hard decision.