Choosing a character isn't as easy as taking your favorite bot from the available selection. You can, if you want, though every Transformer on the roster has its own specialty. You'll want to find the class that works for your play style.
The choice is a little easier in single-player; you're limited to who you can choose and ability load outs are set in stone. The only leeway you have is in weapons. Each starts with a signature load out, though you can assimilate weapons you come across in each level. Choosing which weapons to take and which to leave adds a nice amount of choice. Even if you're presented with a missile launcher, it may not be the best weapon for the job.
Selecting a Transformer in multiplayer is a little more option-oriented. Besides the ability to create your own Transformer (though the options are limited - a minor issue though), there are four classes to choose from. Leaders and Soldiers are front-line troops, while Scouts can cloak and traverse the map quickly. Scientists are a unique class all their own. They can fly, but their perk lists lend themselves towards manipulating the flow of battle. They can repair allies, shift their identity on radar and even drop turrets. Each class has its own learning curve, though overall they are well-balanced and fun to play.
All classes earn experience and can level up, adding newer abilities and weapon options. There's even a special "Prime" class similar to "Prestige" in Modern Warfare.
Combat places just as much of an emphasis on melee combat as ranged, which is the reason for limited ammo. Additionally, it offers plenty of reason to switch to vehicle mode during play. Unlike past Transformers games, switching to vehicle mode is not a gimmick. This is probably why other games have failed and War for Cybertron succeeds. Just the ability to switch into a speedy car adds a load of combat tactics.
I'm as big a Transformer fan as they come, so if you want to chalk my comments up to fandom, so be it. Even without the license, High Moon Studios has managed to put together a great shooter and an even better multiplayer experience.