I find them very occasionally useful. A lot of times people post links with no clear suggestion of what they may be. Sometimes you can figure it out by looking at the URL when rolling over it (if it's a YouTube link or something like that), but if the URL is not familiar, it's handy. Actually USING the damned things is a little cumbersome because they may take a second or three to load, at which point you might as well click and click back.

I'm not opposed in principle (I like Windows Vista's conceptually similar rollover concept, where you can see a miniature version of a different application window by rolling over it on the task bar), but in execution I'm leaning towards "useless bling."