What does Squarespace do well? All templates provided will work with the content you provide no matter what. This may be due to the rigid content blocks you have to pick from - but it is very effective.
What does Squarespace not do well? If you are a power user and want to build in custom features - this is not for you. Customizations are limited to some 3rd party services/products but usually requires an additional payment.
What do you wish you had known about Squarespace before using? I am not sure because the free trial makes most things transparent to the user before actually paying for the service.
Have you used a competitor to Squarespace? If so, how does it compare? Yes I have used shopify and drupal if you can compare drupal to an actual software as a service. Shopify is good at having an online store presence and I would recommend it over squarespace for that. However it is not the best for a simple website that may be necessary for a profile or portfolio.