I'm a single founder (Haystack Software). I wrote/write Arq (online backup to S3). I consider it a success because it pays my bills. A huge side benefit is it gets me into conversations with folks who tell me their business pain. I'm working on products to solve those pains as well.
Having those conversations with the folks with the business pains is golden.
I want to switch my current development job, among other things, because I don't get to interact with the customers having the business problems, and I want to solve them (not to mention it's where the money is, but I honestly like solving business problems too).
Being empowered to solve them should be gratifying too.
I was (and still am) hugely inspired by DHH's presentation at Startup School 08 http://www.youtube.com/watch?v=0CDXJ6bMkMY