Just got through the last fifth level...
Here are my impressions:
The flashing on shooting could be sometimes really annoying, especially when i go crazy and try to shoot every single time i touch the screen(btw, I think the ship should shoot a bit more frequently, say 1,5 times more). But some ppl like the flashing (i personally like to do it with my n900... The graphics are great, exacltly what you are expecting to see, when you hear: Space Invaders!
Although, i think all of the 5 levels are alike, or?
I know piggz you are thinking about creating more levels, but are we going to have a boss at the end? or would You want to keep it simple?..
Just my 2 cents.