It is grindy but for the most part not so grindy that it gets boring unless RNG really fucks you. You can accomplish most individual goals (ie. Farming parts for a particular gun or frame) within a few hours. Also you need pay to win currency for some things but its easy to get items you can sell to other players for that so really you don't need to spend money on the game at all. It's a lot of fun. I probably have more time in Warframe than just about anything else. Except maybe Wow.