Practically Efficient lives on Squarespace 6 now. Why did I move? Well, I figured with as much money as Squarespace is spending sponsoring some of my favorite podcasts, the least I could do is move my most valued web possession there.
Kidding aside, I've been trying to move to Squarespace for quite a while. Unfortunately I kept running into import problems with Squarespace 5. Squarespace 6 fixed all that.
I'm really not a part of the de rigueur "It's time to ditch WordPress" movement. For me, the final straw with WordPress happened a few months ago when a cron process went nuts in my installation. My web host nearly killed my site as a result of it.
Squarespace just seems like the best place for my content right now. It's the optimal combination of reliability and flexibility. And it encourages me to publish more than tweak nuts and bolts.