One of the very best threads I read here so far.
Let me give you a little info and you make up your own mind.
A friend of mine is doing SEO for four years now. After 4 years he makes about US$8,000 on a good month and 3k on a bad month. During the year it goes up and down (seasonal traffic).
overall, he ends with 60k a year and that is after 4 years of work. In the first 2-3 years he was making less then 20k/year.
for the last year I put about 100k into SEO just to get back 200$/month where my current expenses are at about 3k/month. Is it worth it? You make your own mind, but I'm not giving up. I believe it is and I believe a long term strategy has to be in place if you're going to concentrate on SEO's.
Now if you build a site that gets traffic from other sources, then as others said, SEO is just one module of many others.