this is a very well done comedic caper, with brightly outlined characters that are superbly cast (firth, courtenay, diaz and rickman), plenty of funny business (the scene in the savoy where diaz and firth quibble in perfect deadpan over the merits of "the little major" had me in tears with laughter), and the several plot twists were just absurd enough to recharge the comedy without destroying the fantasy. the film opens with a beautiful long sequence, the launching and execution of the caper in its entirety -- which involves the sale of a fake monet painting (the work of a talented forger, courtenay), purportedly owned by a texas cowgirl (diaz) to a bullying billionaire (rickman). this turns out to be a daydream of the caper mastermind (firth). the actual caper is then portrayed in humorous contrast, one miscue or impediment after another. several unexpected complications throw the plot into full improvisation, and as the final twist, in the last scene, we discover that we haven't understood the caper at all. it's all very cleverly put together, and the comedic chemistry among the actors is delightful.
as a diehard coen brothers fan, i have to mention this quirk: the screenplay is by the coen brothers but is "mise en scène" by michael hoffman. if you know the coen brothers movies, the effect is uncanny. at some points you can see the coen visual humor blossom on the screen -- for example, the texas mobile home, the blustery femininity of the texas cowgirl, the natty demeanor of the art forger. but other scenes, such as the prolonged shenanigans in the savoy hotel, seem cut from a late ernst lubitsch caper. this puts a peculiar burden on colin firth as the caper mastermind, who must get punched in the face like nicholas cage in "raising arizona" but look unflappably suave with no pants on running around the savoy hotel. the good news is that firth is quite good at striking the balance ("did you say, room *thirteen* eighteen?") and the comedy amalgam of coen and lubitsch is great fun to watch.