I bought the upgrade to x3 the day it came out. It was a special price of $20. Initially it was not worth it as almost all functions caused it to crash. They issued a patch after a while. I only use it for "playing around" with effects. I don't know if its worth it or not. I certainly wouldn't pay full price for it.
It is fun being creative with the special effects and I am no longer a purist about image manipulation, I think it is just another tool.