Working for yourself, but you should also help others when they are in trouble. Working with others is a good way to get to know who you are working with and have essential experiences for yourself.
This is why I don't actually take on anyone with large workloads anymore. Infact, I only make websites for others if it's a personal referral and a small project. I'm just not motivated for large projects for others, not to mention my time is better invested in my own projects that may yield alot more results (for myself) in the future. And when it comes to working with people, that usually doesn't turn out too well either. Either they end up asking for too much or they just aren't on the same level as I. The last person that I worked with was a friend and he ended up moving in so that we could focus on the work from my home office together. He was supposed to pay his way through work and with our first and only client all he ever did was procrastinate the work. In the end I had to give a refund of the first half they paid up front. Totally screwed. Ended up losing my home. So yeah, if you have the ability to work for yourself by designing, developing, admining, and marketing, then you are best off by yourself unless you have become so large you need to pawn off the work load in manageable doses. Although, thinking past my personal experience, there have been some majorly successful partnerships. Alot of major players out there today were started by partnerships.
My guess is that most people getting into IM seek to work for themselves because they prefer it over working for others. It's definitely my preference.
There is nothing better than working for yourself, you have your own expectations that only you can carry out correctly. There is no disappointment.