I had no real problems with collision detection, using just one X file.
There are other problems with 3Impact though :
* Your main function is called from the 3Impact library (at a set rate, which, last time I used it, it couldn't be changed). This makes it a pain if you want to do title screens, hi-scores etc etc
* Pay for yearly updates
* There are no 2D sprites - everything was 3D, which can make text fuzzy (due to mipmaping).
There are other problems with 3Impact though :
* Your main function is called from the 3Impact library (at a set rate, which, last time I used it, it couldn't be changed). This makes it a pain if you want to do title screens, hi-scores etc etc
* Pay for yearly updates
* There are no 2D sprites - everything was 3D, which can make text fuzzy (due to mipmaping).