Ben Tennyson is an average kid: bullied, is a superhero, saves the world every day- wait, what? Yep, Ben 10 is a superhero. In the series premiere, Ben finds a watch with 10 different aliens on it, and he can change into any of them. Out of all of the series pilots I've seen, this is probably one of the best. This sets up nicely for the rest of the series: leaves you guessing, introduces the main characters, sets up perfectly.
If it's so great, then why did I give an 8 instead of a (Ben) 10? Well, compared to the rest of the series, it's pretty mediocre. The series gets much better towards the current episodes in season 3, and this is just the launching point for a much better series later on.