I just received one as a graduation gift, and used it for 7 days on the Long Trail in VT. I only used it to send "okay" messages, but it did work as advertised. Only times that it had trouble was under dense forest cover, where a GPS would have trouble tracking as well. It would take at most 20 minutes to send a message.

I don't know if it is a good replacement for a PLB, but it does have the added benefit of being able to tell your loved ones that you are okay. The peace of mind for my mother and girlfriend is worth it. I would recommend it to a friend.