Not just a well executed idea.

Its another situation like StackOverflow where they can tackle chicken and egg situation others can't because of their exposure.

Not just exposure, but exposure to the right two communities: many designers follow them, and many coders. These groups tend not to hang out (like biz and coder guys don't...). If there aren't any other intersections, it's a unique competitive advantage.

Perhaps the trick is also in knowing what you got; and what problems others have got.

hmmm... I do wonder if some kind of specific advertising like this was a thought very early on for them when they started dedicating themselves to blogging, speaking to build an audience, as a financial rationalization of what they wanted to do anyway. :-)

Yes, they will get sign-ups because designers and coders know about them. But if I went to ANY of my web design clients from the last 7 years, I would have to say that maybe 1% or less of them EVER heard of 37Signals.

That being said, it looks to me like a bust in the making because if only other designers are looking at your $100 per month "ad", then it's actually working against you.

I was thinking that coders need designers; but you're saying that, in fact, clients are neither.

hmm.. I bet that some coders do need designers (e.g. if the coder was hired first), but it's probably a tiny tiny fraction of all clients.

That helps them get exposure, but it does not guarantee long-term success.

Besides, 37signals weren't born with their readership, they built it.

Nothing guarantees long term success. The point is that part of their competitive advantage is 'being famous'. This doesn't look like a particularly difficult thing to code up, but as stated above, without that initial kick, it would likely not go anywhere.

Also, I don't think the difficulty of coding something up has any real baring on how well it does.