I had a unique idea that I implemented in my part time. The site was featured on Techcrunch a few years back and quickly started making a lot of money. Within a few months a handful of VC funded competitors sprung up. They were able to iterate on the concept very quickly and completely destroyed me within the year by developing a significantly better product then I did.

The moral of the story: Working on your startup part-time is a great way to mitigate risk while you build out your first version. However, if you see some traction quit you day-job and start executing relentlessly.