I know the effects of Pagerank on SEO are negligible, but last year I wanted to increase the PR of a new site on the next update so it looked a little more trustworthy. I paid for a high pagerank blog commenting service (the service provider is irrelevant). 4 x PR7, 5 x PR6 and 10 x PR5. They did a good job back then and I manually checked all the links and comments. 8 months later, out of curiosity, I decided to check the report again. Not a single link suck. Zero. Some were deleted, some sites returned 404s, most had just disabled comments or links in comments. So don't waste your money on them. They might increase your site's PR (and value) but it will just drop again next update.