The acting in the movie is good all around. Scarlett Johansson (who was in her first major role after Lost in Translation) and Ewan McGregor play the parts of the escaped clones very well. McGregor also has to play the "real" person who interacts with his clone which he also does a great job with. Sean Bean plays the main bad guy role as the one who runs the cloning center. He did very well portraying the fact that his character basically had no morals and only cared about money. I personally think the best supporting performance was from Djimon Hounsou who basically plays a bounty hunter tasked to track down the escaped clones. He has to strike a balance between a guy doing the job he was hired for, and not truly believing in the ultimate purpose.
It does have some of the hallmarks of a Michael Bay summer blockbuster, but even if you are only lukewarm to his other movies, this one is worth giving it a chance. It is definitely more than just things blowing up. While it is certainly not an Academy Award winner or something that will keep you thinking about it for days after you watch it, there is enough substance to the story to make it worth seeing.