Well, it is a fine Shrek but no 'must see.' Shrek Forever After is supposedly the final chapter in the series of movies that started off with a classic. The problem is that it didn't play to the formula and so was a little unsatisfying. Last time around, minor characters made the show. This time around they were absent and for no good reason.
The fourth Shrek movie is an amalgam of plots. It starts off with the plot of many 'family' movies with a parent facing what is a mid-life crisis although that usually arises from career issues which Shrek himself doesn't seem to face. Instead, he has been hit by a year (yes, only a year and with no annoying teenagers) of a fairly routine family life and loses it at his triplet's first birthday party. It turns out that this relatively minor issue plays into the hands of Rumpelstiltskin, who, unbeknown to every other character in the movie had been harmed by Shrek's very existence. He takes the opportunity to employ a time honored contractual mechanism where contract law can, if you don't think about it carefully enough, erase you from history.
So the plot moves from a 'family' movie to It's a Wonderful Life which, as is usual in these things, involves an environmental disaster with all the trees dying and not much sunlight. That said, Shrek is more concerned about what has happened to his family than to society and the plot quickly moves on to Beauty and the Beast (with some irony I might add) in that Shrek now has a day to set things right and win Fiona's love. Actually, that might be Groundhog Day. That proves challenging as Fiona rightly thinks Shrek is a dweeb and he doesn't 'get her' the way he clearly did in the first movie. Anyhow, you can guess the rest.
The problem is that the main characters are the focus and the obvious opportunity to bring back the dead but interesting villains from the first three movies is completely missed. So no Lord Farquand, no Charming and no Fairy God Mother. That also meant fewer pop songs although for some reason Enya appeared in a surprising sequence.
All that said, as kids movies goes, this is not at all bad and everyone will like that. It is just not quite what it could have been.