Every time I've gotten to collaborate with another writer it's been a lot of fun. The only problems have been in relation to dividing the work equally, and persevering through conflicting views.
In my view, authors are by nature solitary creatures, and they tend to find it difficult to work together.
Still, I do love that extra set of eyes and brains to help the story attain greater heights than could be achieved alone.
Regards,
Ryan