Not so good. I gave it, I thought, the columns of data it could use for lookups (title, author, and ISBN), but I'm wondering if it's a lot pickier than I thought about format or something. The current status says "Books added : 288, Books unable to add: 2184, Unprocessed entries: 453". Not a very good success rate, and while I do have a lot of books without ISBNs, some of those should match by title and author. The percent with ISBNs ought to be higher than that in any case. Maybe I'll tweak the file and try again, or maybe I'll just stick with LibraryThing--I'm not planning to use either site for 'social networking' anyway, just for reference.
And another thing--it seems to be entering only one book per author. One Balogh (Readerware shows 58, all with ISBNs). One of the Baum Oz books out of 17. Oh, wait, there are multiple Bujolds, but nothing like the full collection. Definitely some glitches in this process, in either my end, their end, or both.