I've only played 2. I think it's a good game, but I rate it inferior to Robotron.
Main reason being there are too many enemies that "just" chase you and you end up spending the whole game circle strafing the arena, whereas Robotron has more varied hazards and enemy types, including some very nervy rng bullet sprays which pretty much encapsulate all the joys of shmupping in their own right.
Now I only played only 3 and not 1 and 2 (and have never seen Robotron really), but I quite liked it. It has been about one year since I last played it though, but here are some impressions.
First of all there is fair amount of diversity in levels. Now of course I think it is fair to say that with the diversity, you aren't going to play each of the levels equally at all (in fact probably most percentage of the play time will be concentrated on few levels for most of the players).
But there are enough interesting and varied levels which play different from what you are describing. Though one of the things that I should mention is that getting all the stars should just be a start (of course unless someone doesn't feel like playing more after that). After that it is really about going for highest score on the levels you like (presumably with added restrictions such as no bomb etc.).
Generally speaking, a lot of the "hardcore" levels are pretty good. One "complaint" perhaps though that one might make is the "start-up" time for a level before it actually starts to become interesting. It does vary quite a bit from level to level though.
Here is an example of high level play on one of the levels (also good example of risk taking for score):
https://www.youtube.com/watch?v=elC6_J2x5uQ
Another good level I remember is "bullethell". I think there were number of good levels (I don't remember much of the levels now obviously).
