I never found Adventure Time to be any good, personally. It's just all over the place and has too much random in it. The people I knew who liked it were lame, too. Most of the people I know who enjoy the show are the kids who think everything related to drugs is cool and act like the show is just one big acid trip. But that's not necessarily the show's fault.
Adventure Time has really evolved over the 2 and a half-ish years it's been on the air it started off as a completely random show, and was extremely hilarious because of it, but it soon started changing it's humor, and now the humor isn't as in your face as it used to be, and now I find myself going through entire episodes without even smiling, but it's not really a bad thing, because the writing on the show is fantastic now as they're starting to create overarching storylines and the show has brilliant continuity.
Regular Show is amazing but as Zephyr said, some episodes can be meh. In fact, he gave it more credit than I would. Some episodes can be downright horrid to me.
And as I said before, the show seems to be headed on a downhill slope right now... I really hope they improve things soon.