It is worth seeing. The point is to watch massive amounts of spectacular destruction. In fact, it is only worth watching in theaters, as you don't get the same experience sitting at home (unless you have an insanely expensive home theater setup). Transformers has always been about 3 things: Blowing shit up, making money, and stupid bits of dialogue. The point was never to deliver a unique story, or any story at all, really. That is what Godzilla got wrong, and Pacific Rim got right. Fuck the actors and their stories. Blow shit up.
At times, Michael Bay gets caught up with trying to make people feel emotional about certain characters and that gets in the way, but those occasions are few and far between, something forgivable when Optimus Prime drives a 2 story tall sword into a Decepticon chest. The sheer levels of destruction make up for those slip ups, and the fact that you dont give a fat, flying fuck about the characters makes death easy to push past and get right to the blowing-shit-up part.
Age of Extinction was no different, and it was awesome in every possible way. It was fun, the story was forgettable, it was expensive and it make metric fuckloads of money, the characters were annoying as all hell, and the amount of destruction made you nearly giggle with joy.
This is my opinion, but one that I know many others share.
Do with this what you will.
EDIT: Also, since good movies with good plots these days (the few of them there are) don't often come with mind-blowing SFX or stunning visuals, I agree with Greg that most movies are not worth watching in theaters, but it is the ones like Pacific Rim and Transformers, the ones that are shitty enough plot-wise to bore you at home, but come with stunning visuals- those are worth watching and shelling out the stupid amount of money to watch in a theater.