I tried them a while back for an emergency phone for my 12 yr old daughter. Waste of money. If I had wifi, calls were barely acceptable, lots of echo and severe delay. If using cellular (VOIP) it was at best poor, many times though it was unusable.
Texts were delayed by up to 12 hours at times.
The phone is now sitting in a drawer somewhere in my daughters room.
I ended up getting her a t-mobil PAYG plan. $3/month for 30 minutes talk and/or 30 texts. No data at all. Works as a great emergence phone, and she can use wifi at home and school, and most anywhere else she is.