JS_Harris - 1:06 pm on Jul 17, 2009 (gmt 0)

Age of site and quality of content are heavily weighted factors, followed closely by internal link structure and proper use of titles. I personally rank link exchanges much, much lower on the importance meter.

An observation, link "pr" is relative. If your niche is extremely competitive and the best sites are PR 8 or 9 you'll need more incoming links to compete than if you are in a niche where the top site is PR 5. Though "PR" per say isn't of much use it CAN be used as a guideline in telling you which niches are busy and which are less so, even then i'm sure there are better indicators for this.

When I see a competing site carrying a PR 8 I know I'll need many backlinks and I'll go digging in their link profile right away.