Squarespace is awesome, but until they offer SSL certificates I can't recommend it to anyone. This is very unfortunate because it's a great product otherwise.
I've personally wanted seamless SSL for all Squarespace customers on custom domains for forever now, and we've put a lot of work into making it a reality. Furthermore, before initiatives like Let's Encrypt, the business/logistics side of getting this in place was a mess.
(Disclosure: I'm the founder/CEO of Squarespace)