Contrary to the spirit of its namesake, Activision's Transformers: Revenge of the Fallen game is exactly what meets the eye: a mindless, mildly entertaining tie-in shooter based on Michael Bay's mindless summer action movie. The game does a good job of implementing a transforming mechanic that blends shooting and melee combat with vehicular action, and it also has a decent grasp of what makes a good third-person action game in general. It's fun enough to use your big robot to blow up other big robots in short bursts, but a lack of variety in the mission design makes it run out of steam before you can blast through all of its available action.
Revenge has separate Autobot and Decepticon campaigns that relate to the movie's storyline. But the missions and locations in both campaigns range from similar to identical, so the only reason to play through both of them is to play as all of the available Transformers. Each of them has different sorts of weapons and, of course, turns into a different sort of vehicle. You can use the transforming ability to pull off a handful of neat tricks, like speeding down the road as Bumblebee in Camaro form, hitting your turbo boost, changing to robot mode and leaping straight into the air, then grabbing onto and climbing up the face of a building like you're King Kong. The controls are generally responsive in both vehicle and robot forms, and the melee and shooting are competent and satisfying enough as well, if a little repetitive.
All the characters you'd expect from the Bay movies are in here, from Optimus Prime and Ironhide to Starscream and Megatron (and, of course, the ones you've never heard of). You'll get to do some flying as helicopters and planes from time to time, but most of the Transformers turn into ground-based vehicles, and all of them handle about the same. Each of the robots on a given side have different weapons and special abilities, but those are mostly mirrored by a character from the other faction, so again, the gameplay starts to feel stale when you finish one side's storyline and then crack into the other (though at least you can play them both concurrently if you want).
You get a marginally interesting progression mechanic by earning experience during missions that you can cash in back at HQ for upgrades like faster weapon cooldown times, more health, and a bigger turbo meter. There are also one-off challenges to complete, like getting a certain number of headshots with a given character, and these unlock some goodies like concept art and several entire episodes of the original Transformers cartoon series. It's faint praise for Revenge of the Fallen itself that my most exciting moment with it was watching "S.O.S. Dinobots" for the first time in two decades--but then, it's tough to beat the Dinobots.
The action is solid enough, but there just isn't much to do from mission to mission. The objectives rarely get more complex than "kill all the enemies coming at you until the mission is over," and when they do, they merely add a simple escort or protection layer to the basic combat. The levels feel a little bit like an open world, but in reality they're just rectangular maps that sort of let you go anywhere, until you hit their artificial boundaries. As it stands, the game is entertaining enough that it's fun to blow away nameless fodder robots for two or three missions per session, but Revenge of the Fallen really could have used a more elaborate world, more intricate mission designs, or something with more substance to wrap its basic combat mechanics around.
That combat at least translates reasonably well into online multiplayer with a handful of modes. They're all stock competitive game types--capture the shard (er, flag), a VIP mode, a control point mode, and team deathmatch--but at least there's a decent number of them. The multiplayer isn't anywhere near as serious as what you get in your Call of Duty or Halo, but there's enough entertainment value there that if the idea of taking Prime online against Megatron sounds like fun to you, it's worth doing a few times.
Actually, that pretty well describes the ideal audience for Revenge of the Fallen in general. If you're just a regular Joe looking for a great action game, you could do better. But if you've still got a special place in your heart for the Transformers series--despite what Michael Bay may have done to it in recent years--the game is fun enough to be worth taking a look at.