I've just had a nice experience with this ;)
I run a non-commercial site. You'd expect that to be more immune to nasty commercial people, but no :P I setup a couple 'hubs' as per good SEO practice... I provided a small description of the pages linked to -- specifically in this case, tutorials.
One was especially bad, and misleading, so I mentioned that fact in my review. I thought it might have been a beginner's naive mistake, so I mentioned it... it turns out the guy knows what he's doing (technologically speaking), and wrote the tutorial to promote his products. I guess that could be volontarily misleading potential customers, or he just may not have been able to get his point accross. He denies this, misses the point and tries to confuse the issue more with a long-winded abusive email.
He threatens a law suit, and asks me to remove the link. I call the bluf, leave the link intact, but remove the description (which works out for both of us). Nothing since...
Surely, the reason for the tutorial is SEO, and to drive the potential customers to the page (undoubtedly he won't admit that in court, which is a bummer). It's then his business to convert that traffic into sales. A link can surely do only good to the company? (assuming the review on the other side is a good one ;)
Surely it's up to the site to convert deep links to main traffic, advertising revenues or sales. It should be an integral part of the site design and flow.
And surely I'm allowed to review other people's work, and post my opinion on my site? Also, if i mention the same link within a forum, say, that would be classed as my personal opinion right, and thereby be considered separate from the site...