I used to really dislike those two. My reason was, i just couldn't see why they landed any roles at all, and the movies they were often in were pretty stupid. Especially Michael Cera. It just didn't make any sense altogether.
But then eventually i changed my mind. After seeing Michael Cera a few more times i decided he can act, so that explained a bit (though not all is addressed!), and the same went for Jonah Hill only in his case i also saw him in a Django Unchained interview where he made fun of himself in such a way that made me like him.
So, i'm okay with Cera, but still don't particularly like him, while i'm also okay with Hill and like him.
I like them both, too. I actually wish Hill didn't lose all that weight. He looks weird now.