Not bad...
This game is certainly not awful, but there are some qualities to it that could be touched up.
I like the customization, and the updates are very nice. The game itself is pretty fun once you get into it. The flash is pretty solid, although Mario and Luigi look a little funky. Nothing that really detracts from the gameplay however. In addition, the modifications to the plot are entertaining, while still being true to the Mario franchise.
The music was good for the first 5-10 minutes, but after that it just grated on my nerves. The sound effects are a nice addition.
However, there are several flaws in the gameplay.
The first one is the fact that you can't shoot straight up or down. That means that if they is an enemy tank, or fireball-shooting koopa directly above or below you, you can't do anything about it, except try to find better ground while they are potentially shooting at you. The bombers are just frustrating. Enemies that you can't kill (Whether that's because they're immortal or because of how fast they fly I couldn't personally determine), is not a good idea in a Mario game, simply because it means you have to allocate some of your focus to making sure that you aren't directly below them.
The jumping controls feel a little off- it feels like jumping on the moon when you go up, and Jupiter on the way down. The fact that you can't double-jump is also somewhat aggrevating, because it means that if you mistime the jump once- you're usually dead. And this is a game that lives on precision jumping.
The ammo I initially thought was going to be a problem, but you get enough extra-shots, that it really isn't an issue.