It's a reasonably clean and intuitive interface. The major problem I'm having with it is its management of circles.
It's fine if you only have a few contacts or a few circles — you can drag and drop contacts onto your circles and hover over them to see who's in them. But once you have more than one row of circles and more than a couple of rows of contacts, it gets icky, pretty quickly. Want to see who's in one of your circles? Hmm, it only shows 13 (the first 13 you added?) when you hover over it. Want to add one of the 100-odd people you're connected with to a new circle? You can sort contacts (though I'm at a loss to explain how "sort by relevance" works), or scroll through rows of 7 contacts at a time until you find the person you want (time-consuming, and feels like a very inefficient visual search) — but a much better way is just to type the person's name in the search box at the top, though this will also show you people with similar names whom you don't know.
However, the worst aspect of circles management for me is that if you have more than one row of circles, the default view is that the bottom third of the screen becomes its own scrollable area and you have to scroll down within that to view those circles (or drag contacts into them). There also appears to be no way of sorting or moving your circles around (for example so that the most commonly used circles appear on the top line to avoid the need to scroll down so often) — this is crazy. Of course, you could delete them all and start again, but that loses its appeal once you have a great many contacts; very fiddly, and you'd have to keep track of who you'd already done and who you hadn't.
I like that the contacts listed on the circles page are sortable, and I like that the icons are visible to permit quick visual search. And broadly, I like the conceit of dragging and dropping contacts into circles. But I really don't think it scales well, as I've outlined here. Also, from a UI point of view. I'm not convinced that dividing the screen into two letterbox-style horizontal panes is the best way to manage the process: I'd rather have seen two vertical, scrollable panes, with sortable contact lists and sortable circles (it could still utilise drag and drop). Scrolling up and down inside two very wide, very shallow panes just isn't that great an experience.