As Ann said if you have the possibily to stay within NTSC since your source is 60/59.94, it's best to do so.
That said, if you're working on a vid in 25fps because all of your other footage is 25fps and you just happen to have this onesource/camera that's shot in 59.94 or 60fps, then well yeah it won't be great but it will still play. You may wish to try a few various time interpolation options to see what looks best (ie. Frame Sampling vs. Optical Flow).
I genuinely don't know what the mathmatical process is for the conversion, but it likely is something along the lines of throwing out 1 and 2 frames in an alternating manner so you're left with only 25 frames every second. This is why of course the result is often herky jerky, because the dropping of frames is inconsistent.
In short, you will see 25 frames per second instead of the 60 frames per second you have. So 58.33% of the frames will be skipped and all motion will look stuttery. As already has been said, stay within NTSC.