Me and my wife run a PR company although she set it up while I was earning "safe" money in a sales job.
The main thing is market research – so many just think their idea/service will work because they like it but find out no one else is interested. And don't be afraid of non unique ideas – doing something loads already do but better is usually the best route.
If it helps at all, we earn way more than we did in Bristol/London, have loads of flexi time with our boy (and to surf/cycle) and choose our clients; we don't work with ones we don't want to. You are also seeing all the money you make rather than your company taking the majority.
On the downside, holidays are hard to take, you can't just leave it behind at the end of the day and the responsibility can get to you sometimes. You also have to do accounts which is hateful but straightforward.