Great pointers!
I've done two graphic novels for other writers and now I write and draw my own comic.
When I was collaborating I always felt like I was doing more work than the writer which was frustrating. They would generally make up for it by taking care of some of the marketing and "getting it out there" social networking effort while I was busy working on the art and layouts.
It was also nice to just to always have a script ready to have something to draw from so I could just keep going. Now, even though my comic is mostly written it still takes a little bit of tweaking while I thumbnail everything out and eventually do the art proper on a weekly basis.
I have turned down a few collaborations where it seemed pretty obvious that it was just some shitty script they got done quickly a few years ago and now wanted to ride a perceived graphic novel craze. Even worse some of those people weren't flexible or willing to let me have any creative input outside of just visualizing their crappy story in a very literal see-say manner (not really my cup of tea).
Every time I've just done something for the money or for anything less than a 50/50 creator owned split I've always ended up feeling resentful that I put way more effort in that the other party.
I would recommend collaborating with either people that are really good at what they do or people that you really like working with and always insist on a 50/50 relationship.
Or just write and draw it all yourself...