The squirrel rat is actually fantastic, very much inspired by the great classic Tex Avery shorts. But the rest of the material... yeah, pretty mediocre to bad. I actually love the first movie, it had great heart, funny characters. The little girl interacting with the animals reminded me of Jungle Book or Monsters Inc. I saw the sequel and it was awful. Caught the third one on dvd and it was worse.
Really, your best bet it to watch the first movie, and then watch only the Scrat shorts and scenes on YouTube.