I took the leap and upgraded my Squarespace 5 site to a Squarespace 6 site! So far I'm really impressed with the improvements and additions to the content management tools. The templates are a lot more attractive and functional than the previous versions, and will be great starting points to build from over time. The import process was mostly painless. It brought all my content and blog posts over, but it did not import my image galleries. That really just gave me an excuse for a long overdue update to them!
But there is some flaky behavior too. The content editing tools will just vanish every once in a while. Exiting that page and returning gets everything working again. Sometimes changes to text and layout simply don't stick when leaving the editor. Image colors can shift slightly between edit mode and live, which caused me me to briefly think the background color of an image was getting shifted in Photoshop for some reason. Also, images can simply vanish from time to time. They appear in the editor, but not on the live web page. The only solution I found was to delete them and create new image blocks. Easy enough to do, but still annoying.
Overall I'm very pleased with the update, and am looking forward to working a lot of the new things into my site as time allows. If you are on the fence, I'd say it's worth updating. And if you are still doing your web sites the hard way by spinning the HTML yourself, it's high time you let that go and let something like Squarespace free you up to focus entirely on your content!