Loved the first one despite some flaws, so I picked up the 2nd one on launch day. Like L4D, its infinitely better when playing with friends or people online. Campaigns and online battles are pretty fun, although some Chapters in the campaign can be frustratingly mindbreaking hard at times. Ranking up the different playable factions to unlock new costumes, parts, abilities, weapons, etc turned out to be more addicting that I thought it would be.
As with the first the best moments come with fighting the giant, insect monsters the Akrid. Each campaign has a few mini-boss giant Akrids to kill, but the climax of each campaign always has the one BIG baddie for everyone to team up and try to take down. In particular the massive ungodly sandworm Akrid in the third campaign is an unforgiving monster from hell, but on the positive side you can blow it to hell and back while riding on the armored artillery train. With a giant canon. Yes.