Level 24 now.
The leveling curve seems to be adding 25k each level now. It was 25k to get from 19 to 20, 50k from 20 to 21, 75k from 21 to 22, 100k from 22 to 23 and 125k from 23 to 24 and now 150k to get to 25.
I have so far not spent any of my own money on the game. I have been getting 100 Coins a night from gym's and have used Google Opinion Rewards to buy some Coins with that money. Mostly I have purchased some Incubators and expansion of Backpack and Pokémon Storage.
I have used a strategy of saving up candy and do a LOT of Evolves at once while popping a Lucky Stone. (I did 58 last night alone.)
The only problem I see with H2H battles is the age of some of the players......mainly small children. If the game had some sort of friend system where you could battle people you know and avoid people you don't, that might be a way around it, but I am not sure Niantic or Nintendo want to risk that.
The game certainly needs to fix the Pokémon tracking system. It also should consider upping the XP rewards for capping the more "rare" Pokémon or scale XP gains based on the CP of the Pokémon captured. They also need to have healing potions drop more frequently from Pokestops as you increase in level. Farming stops just for potions so you can battle is not super fun.
Team coordination is pretty much non-existant in PoGo. This is something I find very frustrating coming from Ingress were we had excellent teams. Frequently I bump gyms up 2 or 3 levels for my "team", with no way to inform anyone.