I have made entire videos with imported XviD or DivX sources. Sometimes you don't have a choice. (I was making a live-action video and was using downloaded sources, but bought the DVD the
second it came out. I
always buy the DVD.)
I just trimmed the to clip I wanted to use in QT Pro, which then saved it as a MOV file. (But who are we kidding, it was still XviD.) I imported it into FCP and while it took time to render, it behaved just fine. For the most part, it wasn't bad. I think I had one time where an XviD/DivX clip was giving me trouble, and it may have been because I was using DV clips in the same project. I can't remember.
It
definitely is not the best way, and I don't do it that way anymore. Now, if I have to use DivX/XviD sources I open them up in MPEG Streamclip and convert them to DV and then import the DV clip into FCP. (And, I ALWAYS buy the DVD when it becomes available! So much better quality!

)