Oh rly? Which plan is that? The Pinocchio plan?
It's not really a plan. It's the $19.99 60-minute, 90 day refill card. With a smartphone, the minutes "triple" to 180, and you also get 180 texts and 180mb data. If you sign up for auto-refill, you get a small discount off that (and it becomes kind of a plan).
If your wireless needs fit that profile, it works out to around $7/month. I've saved a boatload of money going down that route, although I have needed to supplement it with a few data-only refills, which has only tacked on a couple of bucks per month for me on average. It does kind of rule out using any audio or video while not in WiFi range, and you can't yammer on the phone endlessly, but for me that's an acceptable trade-off.