There are several gametypes (team deathmatch, arena, capture point, capture the flag) with a handful of well-designed maps, but Tribes is best for Capture the Flag. Using a combination of "skiing" on downslopes and using your jetpack to push yourself up inclines and preserve your momentum, you can pick up some ludicrous speed. While skiing, the game even gives you a speedometer so you can shoot for MPH or KPH targets before going for a high-speed flag grab. That aspect of the game alone is fun enough to be its own thing, but the real fun comes when you blend that high speed with projectile-based combat.
The game's weapons tend to lean toward the "slow-moving projectile" type (mortars, grenades, "spinfusor discs"), and these projectiles inherit 50% of your velocity. So as you fly through the air, your projectiles' behavior changes slightly. This is a formula for some extremely satisfying shots. When first starting to play Tribes, you will miss roughly 95% of your shots, and your weapons have a slow reload time. But when you are traveling at 180 kph and your target is moving 200 kph perpendicular to you, and you shoot a spinfusor disc perfectly into his trajectory to blow him up right as he touches the ground, it is the most satisfying feeling in the world, and it's a great incentive to keep practicing and improving.
It's impossible to describe the rush of the high-speed, aerial combat in Tribes. Even videos don't do it justice. It has to be experienced for one to understand it.
Second, the class-based system is very well balanced, and each one has its own role to play. There are light, medium, and heavy armored types, with three roles within each armor type. It ranges from the "Pathfinder" (the speedy flag-capper) to the "Doombringer" (the human tank who can kill high-speed targets just by standing in their path as they splat against his armor), and everything in between, including stealthers, snipers, turret-dropping technicians, and long-ranged heavy bombardiers. Whatever your play style is, Tribes has something for you.
And it's free! Can't argue with that. Like League of Legends, the game uses a microtrans model, and the prices are quite fair. You earn "experience" from playing matches, and you can spend that experience on unlocks and upgrades such as new classes, weapons, etc... You can also pay cash to get those unlocks faster. Note, though, that cash only gets you unlocks. Upgrades, on the other hand, have to be earned through XP no matter what. This is an important distinction, because the unlocks are usually not "better" or "worse" but rather give you more flexibility to play a different role. As such, the game isn't "pay to win" but rather "pay to save time."
There are very few negatives to speak of. In fact, only one glaring one comes to mind, and that is the matchmaking system. As a player, you "level up" as you earn experience. Your level has a lot to do with how fast you win matches and how long you have been playing, so it is used as a stand-in for your "skill" when matchmaking. This would be all well and good, except for one big problem: there are only two brackets of players. The brackets are: Levels 1-6, and everyone else.
A decent player can get to level 7 within probably five to ten hours of gameplay. And When they go into that first match at level 7, they are in for a shock as they are suddenly up against level 20 players who have all the unlocks/upgrades and have dozens, hundreds, or (soon) even thousands of hours more of gameplay under their belts. Many players have a great time with the game as they are learning, but as soon as they graduate out of the 1-6 bracket, they become a high-speed target dummy.
Don't be mistaken; this game is very skill-oriented. You have to be willing to get stomped on a bit as you learn the ropes, and the 1-6 bracket is great for that. But you have to develop a certain kind of patience to persevere when you first start playing in the next bracket.
Having said that, it's well worth the time and effort of grinding through those first few hours in the high-end bracket to practice, because frankly, this game is too damn fun to just give up on when the going gets tough. And if you really want to practice your flight maneuvers or your shooting, Hi-Rez has given you the option to play around in empty maps or maps filled with dumb target dummy bots that run around for you to shoot.
Tribes is a fantastically satisfying, high-speed shooter in which every shot you land feels like a skill shot. It may not be for everyone due to its high skill cap, but the level 1-6 bracket should give you enough time with other newbies to get a feel for the game and decide whether it's for you. It's free, so why not give it a try?