I've looked into it and I think it's a great concept. It's just not for me. The biggest issue is the Optimus is a huge step backwards. The fallback cell network is Sprint which sucks in my office and isn't GSM.

I basically do the same thing now, but it's a little more hands-on. The T-Mobile plan I'm on has only 100 voice minutes. Since all my calls are routed through Google Voice anyway, I simply set GrooVe IP to connect when I'm on wifi. This way, incoming calls are automatically routed over wifi. Then I just have to use the GrooVe IP dialer to initiate a call. It's a good way to conserve cellular minutes, but hardly necessary in my case. I have 9 days left in this billing cycle and have only used 80 voice minutes so far.