Once you get to around PR4, I think you start appearing in a decent proportion of the searches that are relevant, especially given Google's increasing de-emphasis of pagerank. I started appearing in most of the searches I'd consider relevant for one site of mine as soon as it got up to PR2. Heck, there are relatively well-known universities that are only PR5!
Agreed. My site went from 0-3 in about 6 months and I did absolutely nothing (and I mean nothing, I just wanted to write so I haven't considered SEO even a little) except write some blog articles.