I've used for many years using $4.99 plan. I cancelled it as I got 3 wirelss phone lines now, no issue asked.
The most attractive features are: China local call back number (as long as China not blocking the web) and its small box carrying back China to be able to call US number with broadband: I did conference call from China using iTalkBB box.
In my mind, VoIP are nowhere comparable to ISDN quality wise. Hence the cheapest wins.