I've used it several months now, and I can say it works very well. You get a list of expired bookmarks, with the reason (404, 501,...), and checking or removing a bookmark is just one click away. Before a recent update, if you had a lot of bookmarks it opened too many connections after a while, making some links appear as they were down, while they were actually working. But now I can't find any problem ! So thanks for this great, useful extension :-)