As a squarespace user, you hit the nail on the head about the great support and the ease of use. Also, SEO wise I have no complaints and articles are searchable in google within 15 minutes of posting.
Other than the obvious downsides of the inability to add third part plugins and enhancements, one of the bigger issues is the inability to customize the URL for individual posts. Squarespace will always make the article title the URL. This is a big problem if you want to be added to google news since they require all posts to have a numeric addition to the url for their crawlers.