Good game overall, but it suffers from the same problem a lot of SHMUPS do, which is the lag buildup becomes awful. In this case, it's particularly bad toward the end of the game.
Problem is you have all this fancy stuff you're trying to do and you need to scale it back. All those moving backgrounds seem nice but really it's just eating up more processing power which could be better devoted to other things, for example.
The last level is extra hard not just because of its intended difficulty, but also because the lag will tend to kill you. Better not ever collect one of those grenade powerups because it'll be instant lag of death if you do.
For those who didn't experience lag in this game, it's not really fair to use your multi-core, dedicated graphics card, huge RAM upgrade several thousands of dollars gaming computer as a standard for if games lag or not. Or rather, if it even lags on those machines then yes it's bad lag, but if it doesn't lag on those machines doesn't mean the game isn't laggy.
For future games it'd really be nice to have an option to turn off effects or at least tone them way down. You can have the gameplay be identical, but just turning off certain effects, and that can make a huge difference.
Otherwise you'll have to go with fewer objects on screen at once to reduce or eliminate lag, which is why it particularly plagues SHMUPs unless they're designed incredibly efficiently.