Squarespace is a popular CMS with simple, visually appealing drag and drop templates.
Like many modern CMS companies, it was first developed as a blog host. Over the years, they have implemented additional features such as domain hosting and payment services. This makes Squarespace an all in one service. For new users, this is a very nice aspect to their business.
I use Squarespace for web design for the above mentioned visual elements. It depends on the circumstances, but having an attractive website is a very good place to start from. It is also easy to navigate, great for inexperienced users. For this reason, I tend to use it for clients interested in a relatively low-level approach to blogging or eCommerce.
I would not recommend it for technically complex websites, but that isn’t a knock against Squarespace. They know their target market, and offer a good product that matches their users’ desires. That being said, it is a bit difficult to wrangle CSS. If you want to micro-manage style elements, there are other options that might be worth looking into, as well.
Squarespace’s direct competitors are WordPress, Wix, and other major CMS companies, though its marketing is considerably sexier than others’.