They're just goofy/cute big-headed versions of some of your favorite pop culture characters. I think it started out as comic book and video game type things, but now there's practically everything ever. Sports, politicians, musicians, etc.
I got one Mass Effect character as a gift a while back, and then 3 Mega Man characters as another gift. It's a dangerous road to go down - if I buy Archangel, there's really nothing preventing me from buying War Machine or Iron Fist or or or or or
It's probably best that I get them as gifts and leave it at that.