I haven't played anything else since it was released. I think Advanced 18 was some of the most satisfying gaming I've done in years. None of the others were particularly tricky except for the one with the turrets in cages (16, I think). That one got annoying pretty quickly. I really liked solving the levels with ingenuity rather than the ones where it was more reacting quickly to avoid getting mowed down.
I don't see the challenges being too enthralling for me, but I'll just have to see. I'm not usually a big fan of 'medal challenges' or high (low) score challenges.