Characters I liked:
-Optimus Prime. He was great, especially in the first one. Could listen to his speeches all day.
-Shockwave. Although he didn't get much screentime, he was a total badass and homages his older incarnation well enough.
-Megatron. A real direct representation of his actual character, especially in the 2007 film. Despite becoming The Fallen and Sentinel's b*t*h I think he was really cool. Merciless and brutal.
-Jetfire. A funny, yet appealing character with a nice background. Pretty cool to donate his body to Optimus in ROTF. Shame he didn't return or something in DOTM. He would've made a great asset.
-Sentinel Prime. The voice fit, the character was good and pretty original. A nice history as well.

Characters I didn't like:
-Starscream. Don't get me wrong, Starscream looked kinda cool as a jet, but I could never really see what his motives were. I could never tell if he was actually with or against Megatron or the Autobots, that being one of his signature traits across many incarnations.
-Bumblebee. BB was okay in the films. An energetic kiddy kind of bot, whom we, the audience, could follow along with from his point of view. What bugs (No, just no BD ) me was how he was too much of a focus point and took up too much screentime, compared to other characters, which IMO deserved to be fleshed out a lot more. I like him, but I don't want him shoved in my face for a good part of the film.

Disappointed and/or underused:
-Some more seekers and identifiable Decepticons would have been nice, though DOTM had some nice alt modes for the Decepticons.
-Shockwave definitely needed more screentime to show off more of his sheer amount of invincible badassery.
-Cybertronian Altmodes. Too unidentifiable as any kind of vehicle, even if they were alien.