If you need reliable service, pay for it.
That being said, my wife uses the free service and loves it. The lag on calls is such that she can't answer in time, but she just calls back. Outgoing calls, and call quality, is just fine. It's just too much of a lag for incoming calls. She hardly ever uses voice, mostly text, so it doesn't matter much (and when she is using voice, most of the time she's initiating the call, like to a customer service or family member or whatever). And everyone knows now to just text her to tell her to call them if they need something more than a text provides.
Well worth the savings for the few (inbound) calls she gets to have to call back.