Well, I thoroughly enjoyed it, though it's very short. Of course, it's definitely modeled after old arcade shooters / beat`em-ups, so it works for me. The "Easy" mode was a good difficulty for me; I can't imagine what "Normal" and "Hard" are like. Either way, it's great, silly fun. Love the look of the game, too, and I normally don't go in for fully 2D games anymore, but this one is very entertaining to watch.
Only gripes: you can't seem to jump down off ledges / platforms (which I mentioned in a previous post), and the bottom screen seems pretty useless. I looked down there a couple of times, moved my stylus (out of curiosity) around on the screen, and then realized, "oh, they couldn't come up with anything other than an extemporaneous use of the touchscreen." I don't know why they didn't at least put all the hub info down there. But those are small complaints. The game is fun as heck, and it's now got me interested in going back to check out the other games in the series, as mentioned by others. This one, though, is gonna stay on my card for a while.
BTW, not sure if it's been mentioned (there are so many responses to this topic), but can someone recommend a good emulator for playing some of the older games on my DS?