Competent, but not amazing.
I'm not quite sure how this film got nominated for Best Picture, nor why Tilda Swinton won for her supporting role. It's a story about a lawyer (Clooney) who is the "fix-it" guy for his firm. Swinton plays the lead counsel for a large conglomerate who is troubled by the choices her job is forcing her to make, while Clooney is realizing that the case is bigger than it first appeared.
None of this is unchartered territory, but the film is done in a highly competent manner, looks great and features very solid acting. It's just far short of an amazing film, and you shouldn't expect it to be that. However, if you are looking for an enjoyable legal thriller with very solid acting, Michael Clayton is a great film for you.