Support has been exceptional even though we haven't had to use it often. In the few instances that we did have to use it to help find sessions or ensure data was being recorded, the support team was responsive and provided quality responses which showed us that our questions were being thoughtfully read and understood.
I have not needed to use FullStory's support, so I guess that's a good thing! There was only one time that I couldn't log into the FullStory app and I quickly contacted them, to which they responded that it was a known issue and that it would be up and running within a few hours (which it was).
I've never had to use their support, but their help articles are a little confusing. Sometimes the information is out-of-date or they return search results I didn't want. For example, I was looking for help on setting up FullStory in our Dev environment and got articles about their Dev Tool product.