I have not used SquareSpace. They are not as common as some of the other options. Possibly this is because they only offer a hosted option, which limits what you can do with your SquareSpace site.
I looked for user complaints about SquareSpace and found only a few mildly retarded people complaining just to hear themselves whine. That's pretty amazing. On the Internet, people will complain if you give them free beer and satellite TV.
SquareSpace does claim to have good migration tools for when you're ready to leave SquareSpace. Heck, this could be a good way to get started!