They're fun to do, but they're harder to sell than solo novels. If you get one with a big-name author and no one has ever heard of you, the chances are the book will sell pretty well and you'll make some money, but you'll do ninety percent of the work and even though a lot of people will read the book, no one will know who you are. If you get one and you are the big-name author, the collaboration won't sell as well as your regular work, you'll do ninety percent of the work, and the few of your regular fans who read the book will complain that it isn't much like your usual work. That doesn't add up. Unfortunately it does.
Collaborations are much more work than solo novels.
They can be much more frustrating. They present special legal problems. They can cost you in a lot of hidden ways. If you're going to do one, here are the things that I've learned that can help you, and the things I've found out the hard way can hurt you. Always determine in advance who will do what and who will own what.