The plain truth was that we needed to bring a product to market (it was a medical device). It was an engineering office were we did everything from manufacturing to visiting exhibitions and talking to medical advisory boards. I even got my own smock and did test runs in hospitals and people believed I was a doctor. I also crawled on my knees in one of my best suits at an exhibition to fix a broken fuse for the device.
It was more a 6 day work week, not the best work-life-balance. After it was established however, work was much more relaxed. I guess the question you should also ask is if the startup is already able to carry itself. If not, time is important and you should expect to work more than a comparable job at an established company.
I am no workaholic, but it was a really fun time. Wouldn't want to do that all my life though. 4 days working is entirely enough for me for that matter.
It was beyond awesome!
Unfortunately, it didn't last for more than roughly 2 years, since I made the insanely stupid decision to run the Romanian office of an international outsourcing company (all went downhill from there).
The plan is to eventually return to this, but in the current environment, it's pretty hard to do.