I'd definitely appreciate seeing something like that too. After a couple of "build it first" failures I've come to see the wisdom in finding your customers and validating your idea first.

I've read several of these SaaS walkthroughs and it invariably seems to follow this pattern:

1. Author has a blog with a large following on some topic.

2. Finds niche in that topic.

3. Makes a product and pushes it via their blog.

4. "Incredibly" their product gets tons of customers.