"My Super Ex-Girlfriend" may have been a clever idea (a super hero gets dumped and goes psycho) at some point, but the finished outcome is far from clever. In fact, it's so devoid of any thought that it's pretty far from an idea too.
The punchlines, if you want to call them that, all ring flat. There's been better acting in comic books, and the special effects are the exact opposite of special. Is there such a thing as un-special effects?
G-Girl (Uma Thurman), a super-heroine protecting New York from bad guys, including super-villain Professor Bedlam (Eddie Izzard), goes on a date with Matt Saunders (Luke Wilson) as her alter ego Jenny Johnson. She subsequently becomes the worst, most psychotic girlfriend ever...with super powers. Matt eventually gets tired of her jealously and general craziness and breaks up with her, enacting a myriad of super hero-sized pranks that garner laughs for their idiocy more than their humor (a shark in an apartment?).
With all these A-list comedic actors one would expect at least a few funny lines but they all seem to walk around in some sort of trance. Even the lines that might have been funny, if the rest of the movie didn't have you so bored, fall flat because the delivery is terrible. Wilson is especially disappointing because his comic timing is usually so on and here it is tragically off.
The real tragedy though is that the film makers decided to focus more on the mostly terrible buddy jokes between Matt and his Vince Vaughn-like friend Vaughn (oh, how clever) Haige, played by "Office" star Rainn Wilson, than on Eddie Izzard's at least somewhat funny and minutely charming super villain. Instead, Vaughn goes around checking out chicks and giving Matt terrible advice that wouldn't have been funny even if they had managed to cast Vince Vaughn in the roll.
It's just all terribly disappointing. A group of funny actors put into a funny premise and given a decent comedic director, Ivan Reitman, produce the exact opposite of what would be the expected outcome. Do yourself a favor and don't take "My Super Ex-Girlfriend" out on date in the first place.