I started with an early version, don't remember exactly which, maybe gen 4. I bartered for it quite a long time ago. Then I had a Sanguinololu, I do not recommend that set, it has very minimal ability to expand. I am now using RAMPS, my only complaint is how easy it is to disconnect the stepper wires. It has a fair amount of expandability.

My next set will be from www.reprapworld.com , the megatronics. It has as much expandability as is currently available(to my knowledge), and is virtually the same price as Sanguine, or RAMPS. The only draw back is waiting for arrival, for quality, I will wait.