I think it all really depends on what you are doing and what your relationship is like. For a couple of years I had a part time business where I bought stuff at auctions then sold it on e-bay. I did pretty good. I put in about 15 hours a week and made some nice extra money off of it. My brother and a friend were both involved in it and we split the jobs equally, got the work done and never had money issues. My friend that was involved was just talking about starting it back up and I am actually looking forward to working with him again on it.
On the other hand I once bought a house with a few friends that we were going to renovate and flip. As we looked for the house and bought the house everything was cool. Once it became clear just how much work it was going to be it became harder and harder to get things done. Just getting everyone together to work on the house became a pain the ass and since I was the only one that was already self-employed it seemed like I had to handle everything since I didn't have a "real" job and I wasn't married with kids. Eventually, we finished it and sold it, but it took about twice as long as planned just to get the work done.
I'm still friends with them and we hang out all the time, but we won't be doing that again
