I work alone.
I did once try to work on a series with a co-writer but it was awful. The co-writer was so unmotivated and would not discuss with me what they wanted changed, instead they just told me "I don't like it". I ended up me taking 100% of the work load, and when my fingers got sliced open and I could not draw, the series died. It was so frustrating. It doesn't help that I later got in a fight with them and we haven't talked to each other for over 5 years....
Maybe if my experience wasn't so awful, I would be more open with working with others...I dunno. If you do decide to go that route, remember that it is a team effort and you need to do your part.