My business is, in many ways (such as measuring by hours), the easiest job I've ever had.
The kind of start-up where you have a fairly large capital outlay, where you have to hire a bunch of people from day one and where there is significant pressure from windows of opportunity and a race to market can easily go out of control in this respect.
The other thing that can turn your world upside down is a real success, or a real setback when you're moving ahead full steam.
I have some other issues with the assumption that any CRUD app can reasonably be said to be racing to market in 2010. Many "real" startups, for whatever definition of "real" one wants, have been technically viable for every single day of the last ten years. The race is also a social construct. The world will little note nor long remember the first mover in groupons (hint: not Groupon).
When you have a few hundred K of your own hard earned cash and a few million of investors money, as well as the livelihood of a number of people riding on your start-up succeeding or tanking and you hit a significant road block there is a very big temptation to dig in and try to save the company at the expense of your private life. Even in situations not quite as dramatic the temptation is there.
It may work, it may fail, there may be private consequences, there may not be. But that all depends on the circumstances, and those are not specified.
Case in point, sometimes my girlfriend works at my office. How is that possible at a normal job? Or taking a week off just to go work at your parents cabin? That kind of out-of-office working is just not possible or a great idea in the normal work force.
One thing I will say: Doing a startup by yourself would be almost impossible with a serious relationship. You need at least one partner to watch your back when you can't be on duty (which happens twice as often in a serious relationship, I've found).
I have been in several startups, and a couple of projects in not-large companies. Much of this time it was a struggle not to be doing the project all the time.
The things that we were working on I found seriously personally exciting, and in fact, the most recent one, my advisor in the effort all but forced me to keep "reasonable" hours. Reasonable converged on 12 hours.