It's been a wait, but Natural Selection II delivers the goods!
There's a lot to NS2 though. Don't plan on mastering the game in one sitting or in a week. Marines can have several different weapons, jet packs and even armored exosuits. You don't get access to them all right away though. As the game progresses your commander will research technologies that enable you to purchase upgrades. Every time you die, you'll start from scratch and must repurchase your upgrades. The better you do, the more you help the team, the more resources you receive. Personal resources purchase upgrades for your soldier, team resources purchase new buildings in the RTS mode... that then enable more upgrades. Add in turrets, repair robots and more and it quickly gets complicated.
The aliens have five different forms although every time you start as a skulk. When you accumulate enough personal resources you can evolve into a higher life form or upgrade your current one with different traits. Traits include armor, speed, camouflage and more. Similarly to the marines, when you purchase an upgrade the resources are lost when you die and you must start over. Carelessness is thus discouraged. Aliens expand through extending the creep (goo on the floor spread by cysts). The alien commander can only build structures on the creep. As more resource nodes are captured, resources are gathered more quickly and many upgrades for both sides can only be researched when additional bases are constructed on "Tech" points. These points become hot zones with both teams gunning to expand and prevent the other side from doing the same.
The multi-player game play is fantastic but requires that both sides work in teams. A good commander is critical as well. Without the right upgrades, the enemy will quickly out pace and overrun you. Don't look for a single player campaign, although there is an exploration mode so you can investigate the maps, structures and units without the pressure of battle. There are also several good introductory videos for you to watch when you wish to further your mastery.
If it sounds complex, it is, but you don't have to master the game to have fun. Start it up and follow your fellows in to battle and you'll pick the rest up quickly enough.