I've been using my GO Plus for months now. It's certainly very practical, when you're spending time with friends, or are just walking / driving around a lot, not wanting or being able to have your phone out all the time. I use it for my daily commute on the bike for example.
BUT: The catch rates are horrible - Pokémon flee more often than they are caught. It is very annoying that you can only throw Pokéballs. You regularly have to check if it is still connected, since it disconnects frequently. And personally I've made the experience that the bluetooth connection actually drains your battery more than having the game open.
In general I have something of a love-hate relationship with it. By now I wouldn't want to play without it anymore, but I definitely think that for the very limited features it offers it should be cheaper.