Word Cookies. I absolutely love it. You can pay for things, but it is absolutely not necessary and is basically only if you feel the need to use boosts that allow you to cheat your way past the level. I think it costs $2 to go ad-free.
It does give you regular popups asking if you want to buy coins, which I admit is annoying, but they are unnecessary to enjoy the game. There’s also endless opportunities to get coins and other boosts for free. I have something like 80,000 coins and dozens of boosts and other than the $2 to go ad-free, I’ve never given them any money.
You do have to have certain word skills to enjoy it as much as I do. If you’re the sort who can crush it at Boggle, it’s the game for you.