Eustace isn't my second favorite, but I do like him. I think one thing about both Edmund and Eustace is that they really do grow and change more than many of the characters (the rest of the Pevensies, for instance), and that makes them rather more interesting.
I'll be curious to see who's cast as Eustace for the next film. According to the books he should be a year or two younger than Lucy, but we'll see; they made Caspian significantly older, after all.