I had ambitions of having a comprehensive post containing all the negatives and the few positives which have been said about Snap previews over the last couple of months, but I simply ran out of time. Here's a fairly representative critical comment:
All joking aside, SPA is not helpful, it's not cool, and it's not winning you readers -- It's bling, a silly little shiney thing designed specifically to increase awareness of Snap.com -- no bad thng, and certainly an shining example of how to use widgets to gain links and attention, but, come on ladies and gentleman, show a little self restraint, show a little consideration for your users. [David Krug's blog]
There's a lot more on Scoble's blog, when he announced that he was turning the previews off. When I last checked, there were 73 comments on this post, most of them critical of the Snap previews.
I really noticed the problem when the Librarian in Black - whom I respect and generally agree with - said that the Snap previews were driving her crazy and pleaded with other bloggers to stop doing it - with strong expressions of agreement from all commentors.