Well it is a rather good set, with some very nice planning, --- albeit a few weerd elements like some of those bent gurders in the factory stage (I always end up falling off those because it's not clear when your at the edge).
However I do rather like the design, some of the interesting ideas like having a full waterfall level, enemy and power up placement and difficulty with progression through the level set, ---- level 1 is easy with many hidden lives and secrets, and some bits of the last level are a right pain (that wind business got me swearing completely).
But it is a very well designed set none the less, ---- and the difficulty isn't quite as steep as Underearth which makes things more relaxed and probably more new player friendly.
I'd personally have given it three or four stars rather than five, ---- but hay, such is the opinion of the majority, ---- and as I said it's certainly one of the better sets out there.
As with all the comments I've written, this is just my personal opinion though, of course.