Whenever I happen to browse the link sales forum here Which probably averages out to once a month, I guess I have some buying to do.
Depending on how competitive your niche is, I think buying links is very important in terms of the speed of your site growth, especially PR and SERPs. I have experienced some good boost after linked from a high PR site. I just wish I don't have to pay too much. But this trend will continue for a while, it seems.