It seems pretty solid. There's not a lot of activity/users I know here yet, so it's hard to say at this point.
I do wish you could follow people instead of just topics because it's people that I'm interested in--which G+ seemed to forget that was what made it so great when they got lost and introduced collections while completely de-emphasizing connections with real people.
I like the interests for finding new people, but it's building deeper connections with real people that I'm more interested in. Without manually going to each person and looking at their conversations, there's no way for me to know what my friends are posting unless I happen to be following the same topic they just posted to (which in some cases may be true, but in others we may not be).