When I'm reading someone's post and come across a link, I'll often hover my mouse pointer over it so I can see the target URL... is it on the same domain, do I want to follow it, etc? What I DON'T need is a small thumbnail of the page I'm going to.
What's the point of the thumbnail?
It's not as if I can actually read the target site via thumbnail and do I really care about their color scheme and columnar layout in advance? For me, it falls into the same category as those sliding DHTML panels that ask me to fill out a survey (ahem, HP.com) or "support" the site's sponsor.
I haven't looked into it much, so it's not clear to me what the benefit is to the site/blog operator... I can only assume that they get some metrics on which links are being followed or "hovered". Whatever... get that another way. One less irritating alternative is the MyBlogLog service that Brad Feld uses on links in his posts (right)... a small popup tells me how popular an outgoing link is and, more importantly, doesn't replace a decent portion of screen space with visual noise.
And yes, I know that you can click the "Options" link in the Snap preview popup and turn them off for all sites. I've done that... a few times... it doesn't "stick" for me for very long (and no, I don't regularly clear cache, cookies, or that sort of thing). Also, while this Lifehacker article suggests that site visitors should go to Snap's web site and "download a cookie", what site operator wants to force visitors to do that? The comments on that Lifehacker article suggest an even better approach for Firefox users -- Adblock to stop them once and for all.
Anyway, here's hoping the recent backlash continues and more sites remove the preview popups.