Great Product Rated 4 out of 5 stars
Simple, clean, reliable tool to check whatever page you want, probably ones you're designing and want to catch your own coding mistakes, etc.
Gets an 8/10 only because I wish it did an easier check on my own bookmarks. It can do a less-seamless job if you export your bookmarks to a file, then check THAT page for dead links. An extra step, but it gets the job done.
A useful add-on, I'd recommend it.