In a word - Brutal! While it starts off with an unexpected BANG, it does slow down and allow the audience to catch a short recap of TF3 as well as let us get to know the new human characters a bit before plunging you head first into Transformer Hell, quite literally. In the process we see a side of Optimus Prime that fans virtually never see, and an old nemesis returns with a vengeance.
From the trailers, we know that Grimlock and the Dinobots show up to fight beside the Autobots, but old school (read as G1) fans might be bummed to find they never actually speak. We also don't get to know Hound, Crosshairs, and Drift as well as we might like, but Bumblebee and Optimus are much as we remember them from previous movies, with some physical upgrades and believable progressions to their personalities given all they have gone through since arriving on Earth. For G1 fans, there comes a bit of confusion as to the blue Autobot's actual identity - though he is called Drift for the movie, he looks like a Samurai straight out of feudal Japan and has Springer's transformations.
There is definitely lots of action, explosions, and things being destroyed as any fan would expect from a "Giant Transforming Sentient Alien Robot War" movie, and this time around the focus has shifted a bit more towards the bots with humans as supporting characters, as it was since 1984, which is as it should be. It IS called "Transformers" after all. The humans still play an important role, but they are closer to being secondary here than they were in the first three installments.
While the immediate threat from two fronts is well established and understood throughout the movie, a greater and more insidious threat looms on the horizon that is only hinted at but never expressly identified - a glimpse in the opening scene and hints from the main protagonist's own words. Thirty year Transfans (those who have been around since the Transformers' beginning in 1984) will know immediately who may very well be making their horrific appearance in the next two TF movie installments.
I will say that some of the action gets a bit blurry and hard to follow at times, but the storyline was easy enough to track for those familiar with the live action movies and provides a plausible continuation of that universe. The caveat is that to understand the whole story, you need to see the first three movies at the very least.
Just like in the previous three movies, sharp-eyed fans will spot homages and Easter Eggs hidden throughout Transformers 4: Age of Extiction, but you may have to watch it two or three times through to catch all of them. But unlike the first three movies there are no extra scenes during the credits. For myself, as a general rule I always stay through to the end of the credits for three reasons:
1. Info on the cast and music in the movie,
2. Easter Eggs during or after the credits,
3. A show of respect to all the people behind the scenes.
It is a long standing tradition in my family that I have adhered to all my life, and I would encourage others to do the same. Without the crew behind the scenes, none of the movies we love would ever be possible in the first place, nor would our favourite movies turn out as awesome as they do.
Over all, I would rate [Transformers: Age of Extinction](movie:206531) at least equal to TF1 (2007) at 4 out of 5 stars, with TF2 (2009) at 3 stars (excessive use of sexual innuendos), and TF3 (2011) at 3.5 stars (somewhat shallow plot), by my own comparison. You might have a different experience with these movies, however.