I have 2 reasons for not like it. The first one is related to it not being part of my browser. I have to hover over the link, it comes up but i have to wait a bit for it to load the image. I don't like that wait.
Also, I frequently use the mouse to help me read (by selecting the text i already read) and this would interfeer too much with that type of reading.
(notice that the first problem could be solved with link prefetching, but i don't like that technique at all)