For me, the day-to-day flexibility and freedom is probably the primary advantage of working for myself. There's no doubt that I'm doing something business-related every single day (7 days a week), but the composition and time per day can vary tremendously. It's 11am Tuesday, and if these clouds clear up and I feel like going to the beach, then I might just go and ask if my co-founder wants to come along! This is not theoretical -- this is how we do it. (There are also 12+-hour workdays sometimes, but there's certainly no explicit calculation to try to make it all balance out. And to be honest, it rarely feels like "work".)
The flip-side of this is that if you value the flexibility of your time, you will spend time building systems that automate your required repetitive tasks. I recently spent a few weeks mucking around in some dirty old APIs to automate a particular part of our shipping & order fulfillment, and while that took a lot of effort, it has now cut a 1+ hour/day task down to maybe 1 minute/day. And now I don't need a PC to do that task anymore -- I can do it from anywhere with my iPhone. And we're quick to roll frequently asked questions back into our product, which reduces the time handling future customers with that question. That's how we can get by with two full-time co-founders plus a small staff doing product assembly and order fulfillment.
This is from the perspective of a bootstrapped (no other-people's-money), profitable, 2.5-year-old e-commerce site. If you have other-people's-money, or if your business model requires an acquisition or is advertising-supported, then I'd imagine that it's an entirely different game. But I love being able to work on what I want to work on on a daily basis, and if that sometimes means screwing around with an unrelated project or just going to the beach and "working on" my tan, so be it. That flexibility is worth a lot to me, and if you're choosing the startup route, my guess is that it's worth a lot to you too.