I got it the day it came out.
This game is full of fart, poo, sex, and puke jokes – perfect for my best friend and me.
As Mnementh said, it’s not super-challenging, but it’s very funny.
The secondary characters are hilarious. My favorite is Gregg the (midget) Grim Reaper, who hates cats. Oh, wait, I can’t forget the Great Mighty Poo, who sings at you. (Name may be wrong, but it’s close.)
I’m still working through it (I’m not the most skilled player in the world, but I do keep trying), and I’m having a great old time.
The best part is showing it to other friends and watching them get caught off-guard by some of the dialog.
I haven’t played Zelda, so I can’t compare. It is definitely in the adventure category, though.
The, um, F word (heh, I’m self-censoring for this post!) is *$%@! in the on-screen dialog and bleeped in the audio dialog. But other, lesser words are used, as is the variant “feck”.
I’d guess the mature rating is for a combination of the language and sexual innuendo. The poo, fart, and puke stuff just isn’t adult-level. Actually, I’m a libertine: I think all of it is just fine for most ages. So keep me away from yer kids! 
As with any new video game, I’d recommend that you rent it first if you’re not certain you want it. You’ll get the idea by the, oh, third room whether you like it or not.
Check out IGN for a preview/review, screen shots, and movies.
Enjoy!
Jeyen