It seems quite useful to me. I differentiate between bookmarks and things in my reading queue. Also instapaper has 2 neat features 1) they generate a text only version of each saved page 2) they can automatically send your 'to read' in-box to your kindle as a .mobi file. Reading a text only version of a webpage on my kindle is ideal.
It doesn't warrant me using another service, just simply creating a new bookmark folder to access from my work, home and laptop pcs