I'm loving it so far. I think I'm about where you are, level 6 or something. Just after you have the snow level where you can't fly.
I never really understood the complaints that it's on rails and all you do is move the cursor around, rotate, and shoot. Those statements are certainly TRUE, but how does that make it a bad game? All it does is make it simple - it doesn't make it shallow. I find that there's considerable depth to figuring out the patterns, changing forms, stuff like that.
It's like Galaga. Everyone loves Galaga, right? That's like a rail shooter, only with even less freedom. All you do is move your little ship back and forth and shoot hordes of oncoming enemies. But some depth emerges in exploiting the patterns, trying for the double-ship, and furiously trying to stay one step ahead of far too many enemies and their shots. Panzer has all the same sensibilities, only it treats you to a absolutely gorgeous and bizarre rollercoaster ride while you're at it.