Saw it last Saturday and thought it was a good movie. Not really sure why the Rotten Tomatoes score is so low. While I still think Shrek 2 is the best in the series, I don't think Shrek 1 or Shrek 4 are very far from each other in terms of quality.
For sure, Shrek was more innovative than Shrek 4, but the Star Trek reboot used a parallel universe to tell its story and it was pretty solid! Anyway, I think if you like Shrek, you will like it.