Oh, and what was the company? It was yet another Web 2.0 flavor of a question-and-answer, poll-your-friends site with a distant revenue model and no user acquisition strategy. Ran if for two years before we shut it down with almost no revenue achieved. Shocking, I know.
Going stealth is more worth it if your idea isn't in the same universe as ye old web 2.0 social network/web analysis/etc. tool. Hint: these aren't real businesses. This is especially true if your idea is simple to implement technically, but not necessarily simple to implement on a whole.
And the idea that "someone is already doing it" is far from a truth. There are plenty of HUGE problems that no one is working to solve - most likely because they aren't problems with technical solutions. The notion that "ideas are worthless" is really only valid if the idea isn't all that original in the first place.