I've been using it as my main browser for a bit now, and for the most part like it. The top sites is now my main page for any new window or tab, and I'm using it as my bookmarks to some extent. The one complaint I have about it though is that it allows some basic editing and rearranging, but no way to manually add a site.

As for the issue with marking threads read, I haven't had that come up yet, though it is only showing the main page for me on the top sites. I could see this being an issue if it moved a sub forum onto the top sites as well.