There is a ton of discussion on the topic link buying and how it impacts your SEO. Living and breathing in the SEO services market, this is something I have been trying to get a handle on. The frustration has been that most information you find in forums and other blogs is speculation. One of the best sources I've found is actually not from SEO services websites, but from Google directly. Matt Cutts has talked about link buying a ton. He specifically says that link buying is okay, but only if done in a way that doesn't impact the search engines. This is not a surprising position from Google.
Here is a short video on the topic from an SEO services team that presented at a conference this January. They take a slightly different position.
As you can tell from their video, they believe buying links is okay if done in moderation. This surprised Chad and I a bit. We had thought that an SEO services company would not take this position publicly. As you can see from the video, they say that link buying alone may provide short-term results, but will ultimately put you back where you started. In our opinion, the reason for this is that Google's algo determines the probability that any link has been purchased and devalues the link juice for those links they believe were purchased to zero. Also based on the experiment reported on in this video, it seems that you will not be penalized for buying a link. Again, the worst that happens is that the link you purchased doesn't count. So the SEO services team that sold you the link has wasted your money. But no penalty. This is consistent with Matt Cutt's position when he states that Google is more interested in removing the people who sell links from their search engine than those who buy them.