I personally have never been in favor of purchasing back links. This is not to say that you couldn't purchase a text link ad for the traffic you get from it. Same thing with banner ads.
However where people make mistakes is they purchase thousands of backlinks at one time. Many times these come in the form of a text link ad, but do not even the end up on websites that are relevant to your own site.
Large search engines such as Google can actually penalize you if you do this sort of strategy. Plus when you think about it there really is no advantage to having your link on websites that are not relevant to yours.
For example if you have a website that is built around the theme of music, why would you want a text link on a website that has the theme of website traffic? That really doesn't make any sense.
There are services that offer to get you thousands of back links for so many dollars per month. They may try to entice you by saying you will only receive so many backlinks per day so as to not arouse suspicion from the search engines.
The main problem with purchasing back links is you are not in control and can end up wasting your hard earned money. So what is a better answer?
A better answer is to get links naturally back to your site over a period of time. This is not the fastest way to do it but it is a better long term strategy for getting quality links.
You do this through article marketing, blogging, social networking, discussion forums, and so on. Any time you can get a relevant link on somebody else's side pointing back to your site in a natural way it is a good thing.
This is a get rich slow approach to building an Internet business anyway. Internet marketers who benefit most from this strategy are those who are in it for the long haul. In the long run you will develop a solid group of backlinks and will be rewarded from the search engines too.