Not bad, and certainly not bad for your first game that you created while learning AS3. It does need some work, however; for starters, the collision detection is a bit off. Many times I would miss a platform when I should have landed on the edge, or I would bounce up rapidly when jumping from underneath a platform. The enemies that shoot bullets definitely need to be changed, as their bullets are incredibly difficult to see and they never stop shooting, forcing the player to take at least a few hits in order to kill them. A player should never be forced to take damage in order to progress... and this brings me to my next gripe. The fire is literally impossible to completely avoid; the player has to take at least a few points of damage in order to jump over even the smallest flame.
Possibly the biggest problem is the fact that weapon damage is unlimited. The health upgrades are limited, so why don't you also limit weapon damage upgrades? If I played the game long enough, I could be walking around with a machine gun that does 999 damage per hit, and that's just not cool. I understand that the game is short enough for this not to be a huge issue, but there's nothing stopping players from grinding levels for extra points to supercharge one of their weapons if they are having trouble with one of the bosses.
A personal gripe that I had was the bonus for getting the skulls... I get +100,000 points for getting all the skulls? If I have gotten all of the skulls, I've likely already beaten the game, so what good do points do me at that point? It would have been nice to have something unlockable, such as concept art, secret levels, anything other than points that were essentially useless.
Definitely not a bad game, but it has its fair share of problems. Good luck with future games, mate.