In short: it's a fantastic mini-series for everyone who loves a good murder mystery. Also one of the first slasher stories ever, written by the Queen of Crime Novels.
In long: There is only one thing I didn't like about the show: I knew who killed. I read the book over ten years ago, but the identity of the killer stuck to me. That took away part of the experience.
Other than that I can't think of any serious flaw in this miniseries. The Brits love Agatha, so they take care when they make the transfer from page to screen. The atmosphere and whole "technical side" was great. The actors were amazing - some big names (Charles Dance, Sam Neil) and some rather unknown names (Maeve Dermody in the "main" role). And of course some eye candy (Aidan Turner ;) )