Personally, although I've read Peanuts cartoons since I was a kid, I never really understood much of the humor involved. Maybe it's like a missing Peanuts-humor-organ in me. I'm kind of ashamed of it, because lots of people like it, even people in my family, but I just never really have. I kept thinking I would understand, and like it someday, but so far that hasn't happened.
On the other hand, I *do* like Garfield -- in moderation.
Especially the earlier ones, which are (sometimes) actually funny. (I haven't read a funny one in the paper in coon years.)
What I like about Garfield is that it's a fat cat who doesn't care what people think about him, and often goes off on goofy fantasty tangents. [*Edit* And I think I like that because I wish I could be more like that myself - unabashed about what I weigh, look like, etc., and not care what other people think of me!]
As for Calvin and Hobbes -- I've read all of them, or all of them that are easily accesible without buying a huge collection, years ago. I know them all by heart, and although I enjoy the humor, I must say it's old hat. I can't read them with anything like original enjoyment anymore, because I know every word and panel. So, yeah, I yawn about Calvin and Hobbes.