Joint Ops is an MMO FPS game made back in 2004 that was NovaLogic's answer to Battlefield at the time. The maps are immensely huge. There are a lot of vehicles, weapons and means of playing the game. The graphics for the time were pretty good too and still stand as being acceptable today. Despite being an old game, it's not dated. Even Battlefield hasn't managed to pull of what this game could bring. Today, there aren't as many players as there used to be, but occasionally you'll find servers with many people playing and mostly, you'll find co-op servers with people playing. The community is still making mods and maps for this game to this day. While the game has lost its major player base, it's fair to say that it has stood the test of time. Battlefield 2's servers shut down due to lack of players, but NovaLogic continues to support the game (although the status of their business remains scarce.) and still has roughly ~100-200 players on all servers combined at any given time.
Joint Ops caters to play who like realism and tactics in games (bullet drop, teamwork, etc.) but also has a slick learning curve. The vehicles are easy to control and the game is just simply easy to get used to if you don't mind investing a few hours into it.
I don't recommend paying $20 for this game, but I would definitely pick it up if it's on sale. Amazon and eBay have sellers who sell copies of this game for about $5 and it's definitely worth it! Words can't describe everything about this game, so you should pick it up and see for yourself! :)