Good multi-player options, but that's about it.
The main flaw of the first game was that the one player Campaign mode just wasn't up to much. The mission design just didn't allow for smooth play (that is to say, they were obviously poorly planned because a lot of things didn't seem to make sense), and I was kind of hoping the sequel would address this issue. Unfortunately, it hasn't. The missions are still poorly thought out, boring and repetitive, and aren't really a lot of fun to play.
Multiplayer is a totally different kettle of fish though, and that's where the Robotech series really comes through. With the addition of online play, the second installment places a larger emphasis on multiplayer fun.
The graphics look very good (as you'd expect from a PS2 game), but the cel-shading of Battlecry has been done away with, meaning Invasion has lost some of the cutesy-look. It's personal choice whether you like the cute stuff or the more serious looking style better.
Overall, Invasion scores well in most categories. If you're looking for some multiplayer fun after a night of beer and pizza, it'll be right up your street. But for those looking for a fun and involving single-player side of things, you may be better off looking elsewhere.