I was very skeptic about this show before I watched it myself. My sister was telling me how good it was but it sounded kind of boring to me. I was expecting it to have some incredibly childish humour in it. However, when I once sat down to watch an episode, I was hooked. I watch it with my sister as often as possible.
I think the overall idea is brilliant and the fact that there are two aliens in the house, gives a good source of humour via conflict and I like the fact that the parent-child stereotypes are reversed so that most of the time the children are educating the parents rather than the other way round. Lots of humour also comes via misunderstandings, cross-purposes and embarrassment (it makes you go: "Oh God, I can't watch this, it's so embarrassing").
However, I strongly disliked the fact that the children changed in the later seasons (I stopped watching at that point, my rating is therefore based on the early series). They should have ended the show rather than try and replace the great characters of Mel, Josh and Lucy. Also adding DJ to the family annoyed me slightly. I personally like those episodes with the original five characters the most.