What does Shopify do well? Shopify is a great plug and play ecommerce solution for anyone from small home based business to enterprise. Many different solution providers have either apps built specific for shopify, API's to be able to develop functionality with, or stand alone services with direct integrations like GoDaddy.
What does Shopify not do well? Shopify doesn't necessarily outline what functions you'll need from each of their tiered offerings without you doing a deeper dive with a solutions rep.
What do you wish you had known about Shopify before using? Shopify uses it's own development language called 'Liquid'
Have you used a competitor to Shopify? If so, how does it compare? Yes, Squarespace, BigCommerce, Magento are all direct competitors, each has their own ups and downs in comparison and is best to discuss with a representative for what specific functionality would be needed for your commerce business.