I use Squarespace for my own website, and to build websites for my clients. I love it because not only are the websites beautiful and really functional, but they're also so easy for clients to maintain. If they want to change some wording, add a new review, or upload a blog post, it's so easy and much less overwhelming for them than Wordpress and many of the other website platforms out there.

Everything must exist within the Squarespace ecosystem and they work hard to ensure you work within their ecosystem.  It is absolutely great for webpage building, but in regards to ecommerce, you are quite restricted outside of basing product display.  If you want to add customized aspects to your site, expect to hire someone or be well versed in coding, ready to spend a lot of time making everything work.  

They offer a basic marketing and email campaign platform, but if you're looking to scale up your marketing campaigns, you're quite limited and it's difficult to port everything to mailchimp or constant contact.

Most frustrating, they offer great analytics reporting, but you are unable to export the data.  

The main reason I like and recommend Squarespace is that it's an all-in-one provider. If my site ever went down, it was back live again, before I even had a chance to contact Squarespace. There was never what I had suffered before, when the website builder blamed 3rd party apps, or the hosing site, which can make problems very time-consuming to fix. Also, it is designed so that non-coders can build and update the site easily, so I didn't have to keep calling a website builder each time I needed a slight modification or addition.