My impressions/details:
OK, well having spent the past 4 or 5 hours merely playing "escape" over and over again, it's a ton of fun online. I only had 1 laggy match and 1 disconnect the whole time, as well. The servers are very lively, and I love it so far.
The escape match is 2 rounds, and each team has a turn as marines and xenos. As the marines, you have to make it as far as possible out of the place, or escape. Then you switch. Whoever got the farthest as the marines wins. The marines only respawn at a couple of checkpoints, where as the the xenos will respawn every 5-10 seconds no matter what. It sounds hard, but the xenos are significantly less resistant to damage in this mode, and the marines can attack with a handgun while downed (until they bleed out or get murdered). This mode is a lot like Left 4 Dead, but on smaller maps. It was the reason I bought the game, and I am really happy with it. It controls pretty well, and I have no issues.
Extermination is similar, in that you switch sides half way. The point is to destroy as many egg nests as possible (or defend them). Whoever destroys the most in the time limit as the marines wins. I have not tried the survival mode, team deathmatch, or the offline stuff yet, but it's all pretty self-explanitory.
The upgrades you get when you level up are neat. Marines have a variety of weapons and you can have 5 different classes to choose from set up. There is also a surprising amount of customization for the marines (male, female, races, 4 face types each), and tons of little things to unlock (like camo, face paint, details like having playing cards on their helmet, etc). The xenos are more original, as they have a wide variety of different attacks, wildly different appearance options, and there are 3 different breeds to customize with their own skill-sets.