We have been using them for over 6 years now, they help us deliver great experiences through our site by being able figure out what works well or not so much in each page we offer users access too! They are very helpful at figuring this stuff out when you're new into something like that because it can be difficult otherwise - sometimes I feel there's more than one way some people could think about things as opposed just having 1 clear answer which is frustrating (but also good though).
There aren't any downsides yet but if anything ever did come up then their customer support would fix those issues quickly :) Have used other services before where everything was fine until suddenly someone needed assistance from another department/company etc., who were unaware these problems existed within my own system :D So far nothing major has happened since starting off working together witih thier team.