What does Squarespace do well? I found that Squarespace had by far the most polished, modern templates for my specific use case (which was a personal portfolio). Once I got going in the editor, I was able to launch a really solid personal website in a couple hours, tweaked to my likings. The fact that I can build for web and easily adjust the mobile layout was awesome.
What does Squarespace not do well? It took a bit of time to figure out the editing system, but the documentation was really useful. I also would've liked an easier, faster way to look at all the different templates available with my content. Lastly, pricing is a bit high - I opted for the monthly plan which was fine in this case, but was more expensive than I thought it would be.
What do you wish you had known about Squarespace before using? I thought it would be more drag and drop, but I was actually happy it wasn't. They did have some guidelocks in place so I didn't completely destroy the template, so I was thankful for that.
Have you used a competitor to Squarespace? If so, how does it compare? I tried Weebly and Wix and found that Squarespace had by far the most professional templates and the best editor. Pricing was pretty close on all three.