This come out in the middle of me needing something to keep my mind off the remodeling delays.
Kyle got it day 1, and has been out pretty much every day with friends capturing pokemon or gyms.
I got it a few days later out of curiosity:
On one hand, it's easy to pick up, and has gotten me out walking every night (though some of that is just me figuring it's good for me, picking places to go walk that might also contain pokemon has been kinda fun).
The game boils down to 3 areas:
Capture, stops, and Gyms.
Capturing is pretty easy, just wander around and see what pops up and start tossing balls. No chance to weaken these wild pokemon like in the core series. Also unlike the series, you shouldn't get attached to any of them, a stronger one will be by later, and it's far more efficient to just replace your pokemon than to power it up. Maybe that will get changed later, but it's rather discordant with the themes of 'Pokémon" core franchise, which emphasized the special bond of raising your Pokémon.
My one complaint on the capturing aspect is that if you stray too far 'off grid', there will be nothing. Go hike a trail in the actual wilderness? Nothing. Walking through downtown, you'll see hundreds. Counter-intuitive.
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Stops are the most crucial element of Pokémon Go. They are where you can collect items (balls, potions, revives) essential to actually playing the game. However, thanks to the "Lure module" (available for premium currency and microtransactions), they are also the most efficient means of CAPTURING. Parks where 3-4 stops are clustered together within a small space have become gathering areas where people will slap up a lure on each of the stops, sit down, and just collect pokemon as the lure spawns them. I'm aware of approximately 100 people gathering for an organized 20 stop lure party Saturday...(I'm pretty sure that's overkill so severe you wouldn't be able to capture as quick as they are spawning) Plus, the stops are there for when you run out of balls.
Businesses are using this to draw in customers. Clever thieves used this to draw in victims. And it's totally counter to the whole concept of getting up and off your ass and out in the world. The kids and I witnessed two such lure parties last night as we walked 5ish miles down a trail. The ones sitting at lures might have better pokemon, higher levels, and all that. I prefer the activity mixed in.
Your mileage with the game will depend on how easy it is for you to access stops. Our neighborhood has 3 stops, only one "close" to us, the other two are in an apartment complex (along with a gym there as well). Obviously, those in the apartments have an easier go of it than we do. As it stands, the 3 stops is not really enough to 'support' normal play (unless you live right on them and then can get items from them every 5 minutes), so I have to make a venture somewhere with more stops to stock up items every few days.
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Gyms. Ah, THE only area to battle. Capturing, or placing one of your pokemon to defend a gym for your team allows you to get some free premium currency...once every 20 hours. Some people seem to think this means you need to DEFEND that gym for 20 hours. (An apartment building near us is rather adamantly opposed to anyone attacking it, to the point of hollering when someone tries. Man were they pissed when a bunch of 20 year olds didn't care about the hollering in the least and took it down anyway) However, all that matters is how many you are defending at the moment your timer comes up to claim more coins. So, the most efficient means is to just capture a bunch right before you claim your goods for the day.
My pokemon are not that strong, and most gyms will have stronger defenders, but the system favors attack to the point where even I can tear one down if I set my mind to it and there's no one actively trying to stop me.
Actual battle mechanics are very basic. Pokémon only have 2 moves instead of 4. Status effects are not present (or maybe extremely rare given my sample size. And it's a real time tap fest rather than a semi tactical turn based combat like the core games.
As such, when it come time to capture Kyle's gyms for the day, we swung by the cemetery. Realizing 'defending' the gyms really don't matter, instead of using his best choices, we made a joke of putting ghosts into the cemetery gyms. Did the next person to come by and see the gym get the joke? Who knows.

Bottom line:
It's a worthwhile distraction right when I need it.
"Pay to win" sort of exists with the lure items.
Your mileage will depend on how easy it is to access stops in your area.
I'm not sure how long it will hold my attention to be honest, kinda depends on what upgrades they manage and how quick.