The original Final Destination was exactly that, original. The second one followed quite well, as the characters were all interwoven into the fates of the people who got off of flight 180.
However, by the third film, the characters seem to think "Oh, it's happening again." With no second thought, they assume that that is what must be happening. They hardly even seem concerned.
The deaths are ok, but nothing to the first film. Some are prodictable, like the guy in the gym getting killed (I can't even remember his name, it had that little effect.) Also, the actors look the part, but don't play it well at all. As I said before they don't seem terribly shocked or even upset when their friends die and they know that it is happening again.
The only really good part of the film is the end, which I wont spoil for people who haven't seen it. It is quite clever though.
To sum up, only really worth getting if you're a die hard Final Destination fan. Horror fans are likely to be bored.