I thought it was great, I took my 6 year old son to see it becuase the rain stopped us from enjoying the pool. I loved it, thought it was Pixars best movie in years (Incredibles and Cars was garbage IMO). The whole ecological lesson was good because it was a significant part of the movies story, yet it wasn't overly preachy, but can you really disagree with the point the movie is trying to make? I can't.
Also, I second that Portal simularities, I thought the same exact thing a few time during the movie.