For plain text it's perfect, but if there are alot of links close to each other, like tables for example it's not as good.
You would have to purposely go around the links to not get a hovercard, it can get quite frustrating/annoying.
Example:
http://en.wikipedia.org/?title=Valentino_Rossi#Career_statistics
A motorcycle in that table could also be displayed once, instead of over and over again while it has the same name.
It should be limited in my opinion, but it would take time to adjust it properly since it's not a simple thing to get right.