Once installed in your browser, the add-on still lets you see previews of videos from search results on Google and Yahoo. However it now throws in video previews on FriendFeed pages, and adds a neat timeline view in YouTube that lets you skip to later parts of a video just like DVD chapter markers.
I had the Greasemonkey script installed on my machine last November, but recently turned it off. It directs links that would normally go to where the video is hosted to a special page on Videosurf where it has been re-embedded. This can be useful for some videos, but if you like reading comments and leaving ratings on the original video page it's adding an extra step to get there. Worth noting is that the Firefox iteration of this tool no longer does that.