I went to see The Ladykillers this evening. If you set your expectations sufficiently low, you are unlikely to be disappointed, and I have to admit, it was much better than I had expected. The transposition of the plot from England to Mississippi is fairly literal; the British class system is replaced with God and race as a driver of tension between the characters. The 15 certificate is probably due to one of the characters, who streams foul language throughout, which is irritating and entirely tangential to the plot. The tweaks to the story seem reasonable; many of the gags have been left more or less unchanged, with mixed results. It seemed that there was also a passing reference to A Shot In The Dark which, like the original The Ladykillers, starred Peter Sellers and Herbert Lom.
Ironically, I thought the musical excerpt from Boccherini's famous Minuet was one of the biggest mistakes in the film - it didn't fit the scene nearly as well and conjured up images of the original. The Professor character doesn't work as well either: although Tom Hanks plays the role well, it's just difficult to see his criminal character masquerading as an academic, and his reign of discipline on his crew seems to give him an incongruous authority; Alec Guinness' character is much darker and more naturally controlling.
On the whole, a good film: not as good as the original, but not as bad as some critics would have you believe either.