Decent.
The game is fairly well balanced once you get the hang of it... the key, I have found, is mostly in deflection moreso than brute destruction. To that end, a number of useful tools have been provided...the cost of your available equipment seems to be mostly on par with its usefulness and the rate at which cash comes in....cheap enough to be available when you need it but expensive enough to make you think twice about deployment...and not so affordable as to be overpowered.
The music is a decent loop...gets kinda old after a while. Sound effects are pretty much standard. Graphics are by today's standard, average.
My biggest problem with the game was that I got bored. I played a couple times, got a feel for how everything worked...
then racked up 12,000 points without a single asteroid reaching Earth...defenses still going strong....and I just got bored. Given enough time, I'm sure...things would have become spread too thin and I'd have ultimately lost but I didn't feel like sticking around to see how long it'd take.
The game is a good way to kill about half an hour at most. It has a bit of decent replay value but only for about a 15-minute time-chunk.
In future versions, I'd suggest adding different modes for play to add variety. Perhaps speed rounds where everything starts coming at you a bit faster.
...upgrades....the way this game is set up, being able to upgrade individual units may be a bit too cumbersome (except for perhaps the Bastion), but being able to upgrade unit TYPES could be useful. ..increasing range, knock-back, damage...perhaps being able to decrease the shockwave effect of asteroids blowing up near them and moving them around.... nothing too comlex..the simplicity of the game is charming and one of the selling points I think. ...As is though, it's just slightly TOO simple.
As I sais though, all in all, a good game. 7 stars. Please make more of them :)