Cruel Intentions was (is) an excellent film, Gellar plays an excellent role as the two-faced evil super-bitch and the film is generally excellent, aswell as something you want to watch more than once.
Cruel Intentions 2, in my opinion, is awful. The actors really aren't very good looking which instantly takes the point away of the "beautiful rich kid" image. What's more, the plot is really rubbish and the corny evilness is WAY overdone. Secret popular societies in attics? Puh-leese...
Cruel Intentions 3 is a LOT better than 2...I wouldn't say it's quite as good as the original, but definitely worth watching. It's got really good looking people, several super-bitches...I think the characters in here are more evil than the other films. The plot is adequate, though again you are conciously aware that this kind of manipulation couldn't occur in real life, things don't slot into place that easily - whereas in the original, whilst this was still true, you didn't really notice it. A good film though.