Looking for feedback on new game: "Incoming"

Hi, I’m working on my second Flash game and it’s now in a mature playable alpha state. The objective is to shoot down incoming enemy threats, similar to the 80’s arcade classic “Missile Command”:

http://lab.wx3.com/incoming/

I’d greatly appreciate hearing some feedback, specifically:

[LIST]
[]What are your general first impressions?
[
]Is the interface clear? Are all the elements of game play obvious or self-explanatory?
[]How is the difficulty setting?
[
]Is the game fun? Is this genre of game (arcade shooter) one that you normally play?
[*]What features does the game need to be released? What features would be nice?
[/LIST]

Thanks in advance for any comments!