I'm disappointed with the designs, they look horrible in my opinion. I know that expecting good looking mecha designs from Hollywood is kind of stupid but these guys have enough money to hire the best artists in the world and the results are underwhelming at best.
They were so bad in Avatar that I couldn't believe my eyes when I saw them for the first time, those military mecha looked like garbage. That kind of stuff was done much better in an anime like Gasaraki.
Raiden (robust, utilitarian looking and sexy as hell)
(Cockpit is sexy too)
Pacific Rim designs are boring, they don't look cool at all. The alien suit that appears in District 9 is the only good design that I can think of right now, that design looks like it was inspired by Junji Okubo's work and they should do more of that. Bland designs like Avatar, Matrix Revolution, Bayformers and Pacific Rim are a waste of time IMO.