I think they are very useful, but so far, they have more often "gotten in the way" then "been useful" for me. Sometimes my cursor just happens to be accidently hovering over a link a little too long while I'm reading another part of the article, and then it literally gets in the way. After all, many articles have a lot of links on them; you can then easily "accidently hover" while reading.
This could be solved either by raising the delay (you'll be happy to wait one full second to get more info, no? while that would reduce 'false positives'), or by changing the 'tooltips' so that they actually start as a oneliner, and they only expand once you mouse over the tooltip, or even click on it.