According to a survey, paid for by GM one assumes, apparently Transformers helped boost GM's public profile favorably. The Hollywood Reporter said Bumblebee emerged as the film's favorite character followed by Optimus Prime (91%), Shia LaBeouf (83%, Megan Fox (82%), and and the rest at 69%.
To GM though, probably the most important bit was 70%-90% of the respondents indicated they would more likely buy a GM car featured in the movie. "Moviegoers' opinions of the GM vehicles also shot up drastically, with a 144% improvement in their opinions of the Solstice -- which has only been on the market since 2005 -- and a 72% improvement in audience views of the Camaro. Opinions of the Hummer H2 and the GMC Topkick improved by 65% or greater."
From the article, "92% of moviegoers said the Camaro was the right fit for the role of Bumblebee, 80% said the Hummer H2 was a good fit for Ratchet and 76% thought the Solstice and Topkick were the right choice for Jazz and Ironhide. Bernacchi said that regarding automotive integrations in films over the past five years, typically only 46%-62% of moviegoers have rated the vehicles as the right fit for the roles."
However, despite the improved profile for GM due to Transformers, it apparently has not resulted in an increase on sales or web traffic that they can determine. Still, considering how unfavorably American made cars are viewed compared to Japanese, any improvement is a good thing. It was also indicated that GM is hoping to participate in the upcoming sequel. (source)