So the Plus came out but it doesn't seem that great:
https://www.reddit.com/r/pokemongo/comments/52ulu6/pokemon_go_plus_the_plus_uses_only_regular_poké/
At the moment it only uses poke balls. If you have great or ultra balls and dump away all your poke balls, it won't use them.
At least it tracks your distance while your phone is locked. Should help with battery life.
I got one for my girlfriend and myself to share for when we go on runs or drives, but you obviously don't want to stop to catch/swipe.
I will say this:
It has a lot more functionality than originally suggested.
It has the light codes to tell you:
-how many items you got from the stop
-if your bag is full
-if there is an unseen pokemon
-if you are out of range or losing connectivity
-if you catch or fail to catch a pokemon
I am sure there is more but have not used it much.
As far as the distance tracking goes, it is greatly improved. When your app fails to connect to their servers temporarily, it still seems to track if poke go + is still on.So much so that people have been convinced it counts extra distance when all it does is count it more accurately (1.6 x when i have tested it with the missus). I even open the app up to find that there was the loading symbol in the top left suggesting a loss in connectivity, but as soon as it finished loading i teleported to my current destination, it added the distance between.
I will also mention that it seems to be less effected by speed changes, not just from the accuracy of the count but my drives seem to count more too.
Battery wise makes a big difference too once locked. It does not take up much more than running a locked phone with google maps in the background.