I have been on their invite list for ages, also got 2 invites but gave them away (other sold it for $100 or something which I was not very impressed about) So why I did not take the offer?
As someone already mentioned, limited customized phones that will work on only their network, fine, they are there for the business. Still I was not impressed and that is the reason, I am still sticking to GSM. Second, more important point, whatever software-hardware combo, they are using, is not well optimized. The forums were flooded with problems. RW was quick to try to help the users in the beginning but as the load increased, they started censoring it. A big NO NO for me. So although the plan looks great on the paper, if your call is dropped as you go from wi-fi to cellular network, its not fun.
If someone really wants to save money, visit howardforums.com, the guys are expert in this business. I have HTC One X, Airvoice wireless ($10 for 3 months, 10c/min or /text), data optional at 35c/mb or something but I never use it. I use Cablevision's wi-fi wherever its available. Otherwise, install apps like Viber, Vonage, Tango and encourage your contacts to use them. As wi-fi becomes omnipresent, the data charges (and carrier's greed) must reduce, otherwise they are doomed for sure.
Offnote, there is a rumor that google is trying to venture in this business with TMo, not too sure how disruptive that alliance is going to be.