I'm guessing that you'll need to have your phone on you to get mileage, but it should still alert you of things like Pokemon, Pokestops, and hopefully gyms. One thing I'd also hope it does is alert me when my eggs are hatching. It doesn't need to be specific or anything, it just needs to flash the little light, and I'll see what it alerted me of.
Anyways, I think this is WAY too expensive for a mobile game, even if it's a major hit, and an AR game such as this one, I don't think that this is worth $34.99. If all it does is alert me of Pokemon and Pokestops like it says, then I'll stick to bringing my portable charger along with me, and playing with the screen on full time.
Also, if you live in more urban areas like a lot of us do, then this isn't worth it. Pokestops are everywhere, at least where I live. You can't move a block without there being a stop, and they're still frequent in parks, trails, and monumental areas, where most people will try to catch their Pokemon anyways... In the end, you can still get a portable charger for like, $10 bucks, and will keep your phone on for long 4 hour walks.
And Battery Saver in the 'options' of the game helps too. It makes more of a difference in areas with less Pokestops such as suburbs, or rural areas, in which case the GO Plus will help you. But since most people these days aren't about to try and go out to the suburbs to catch some Pokemon, and most people live within an hour of a bigger city, it's recommended if your one of those people in the middle of nowhere.
In the end, unless you live in a rural area, this isn't worth it. $35 is just too expensive for something that can solved by bringing a portable charger if battery draining is what your worried about. If they wanted this to sell more, they should've made it maybe 20 bucks. At this point, you see older and newer fans having them instead of just those hardcore, never sleeping Team Valor people (yeah I went there). They'd make much more money in the end if this was reduced by maybe $10-15.