Minimal improvement over the first
This is more or less exactly the same game as the first. Nothing about it has been improved. There're still problems with the charged-up attacks not always being worth waiting for, and the same powerups still appear at the same places every single time. Sure there's a new background, and new enemies, but other than that it's exactly the same. There's not even any new weapon types. The first one was pretty good, but this one doesn't really have anything that you didn't have in the first.
It's a decent game, but it's just entirely too similar to the original. At least some improvements would've been greatly appreciated, and would've added tremendously to this. It's not really much of a sequel.