How effective is buying text links?

There are tons of brokers out there selling PR5 text links for example. What does it mean and what ROI can I expect? Will it effect my SERPs or my PR?
__________________
Webmaster of a 10Mio$ Company. If you offer SEO services with great ROI please PM me.

I think everyone should prefer free link building techniques.

You can get massive traffic and high rankings by buying links.
You should buy links from high quality and well managed sites.

Before buying see that where you link is a related or similar niche...or the link would just be considered as SPAM...and will not give you any benefits..