It is a relatively easy type of game to make actually. You could probably make a similar game in Flash with about 200 hours of work. I know that I made a similar game when I was a kid, and after some research I can see that is is still pretty simple.
Maybe I will get back to work on the project I had planned and write the modern version of my old game, "2012", although since the year is coming up pretty quickly and technology is nowhere near what I had portrayed in the original game, the title would be kind of wierd (plus I am not even sure I would finish it before 2012 even ended).
If I do get back to the project, I will post a free download for AT to download and try out. It is in a similar style to Jets'n'Guns but it scrolls vertically instead of horizontally and takes place in open space.