I've worked with a partner on several screenplays and it has always been an incredibly rewarding and creatively fulfilling process. Having a partner forced me to keep deadlines; after all, I had made a commitment to this person that I'd finish X,Y, and Z by a certain day. Also, our strengths complemented one another very well.
I worked twice as fast (we were separated by an eight-hour time difference during our last two projects, so while I slept, my partner worked and vice versa). And I was always excited to see what my partner had added to the story each day.
I will say that you have to choose your writing partner VERY carefully. It should be someone you respect, with whom you can work, discuss, argue, laugh and still be able to look one another in the eye at the end of the day . . . or the chapter. I feel very lucky to have found such a remarkable writer with whom to work, and I'm not sure I would necessarily reach out to someone else and attempt to create the same kind of partnership.