The goal of having each level feel different is definitely achieved, usually by giving you a new item (often replacing an old one) to play with. Some levels are interesting but I find that progress is quite boring. I usually lose a good deal of health the first time I play through a room while I'm working out what needs to be done to get ahead, next room in I die. Back to the start, breeze through first room, low health from working out next room, die in third room. Breeze through 2 rooms, work out etc. So I end up mindlessly playing through shit that is now boring, to get to the interesting part. I did play on hard off the bat though, and perhaps that is lowering the amount of save points in the level.
The boss fights also sometimes suffer from the same problem. Like the one I'm on now, I learned what to do to damage it, that worked for two hits, now something different to damage it! Game Over a few times until I get the third hit, now it's a new thing again, then again. It's very repetitive, especially as it effectively has a 'vulnerable time', so it feels like waiting around until I can get to the bit I actually want to play. I guess they are just too long.