First of all, I have to thank Steve for his Squarespace review. It spurred me to try Squarespace and I have to say that I can't imagine any of the other builders I've considered working as well. His review is spot on; I'd have given Squarespace a 4.5 if I'd been able to.
I normally use Wordpress to build websites and have also used web designers' own CMSs. Squarespace is a far more enjoyable experience. The interface itself is a joy to use - once you get the hang of it. I didn't find it very intuitive, so there were a couple of frustrations at the beginning (e.g. you have to delete the image that comes with your template in order to replace it with your own - I found that disconcerting at first).
The reason I went for a 4.5 instead of a 5 are the templates and a few other limitations, which I'll explain.
Yes, the templates are beautiful, but they're all in much the same style. Great if you want a website that looks like the cover of Joy Division's Closer album, but limiting otherwise. And there wasn't that much that I could change within the template, apart from fonts, padding and colours.
The basic version ($8 per month) lacks capabilities such as a contact form which takes the user to another page (essential in today's content marketing world). And there aren't any anti-spam forms either.
Perhaps I haven't mastered it yet, but the blog interface of the template I chose (Hudson) is very limited. For instance, there don't appear to be any social sharing buttons (how can that be possible on a blog?) and I can't find a way to add thumbnail pictures to the blog's home page.
But overall, Squarespace has been the best interface I've ever used to build or manage a website. Possibly a lot of my issues are things that a more technical person could sort out. Overall, I'd definitely recommend it to anyone with a little familiarity with building a website.