VWO is used within our digital department to run A/B tests and to personalise our website for visitor segments. I personally also find it useful to watch recordings of visitors that have dropped out of our ecommerce funnel. VWO makes it easy to select these recordings, instead of having to manually go through a great number of irrelevant recordings.
The A/B testing functionality allows us to validate whether specific changes that we want to make to our website actually have the impact that we want them to have, rather than just going by gut feeling.
It has made it easier for the unexperienced to make changes and run a test. I think this also has the downside that people underestimate what they are doing on the website. The very first test ever that I tried to make was erroneous due to my editing dynamic content using the WYSIWYG editor. Making the tool easy to use for non experienced people is a double edged sword.
I used Google Optimize when it had just launched. It was therefore not yet a competitor to VWO. I haven't used it in roughly half a year time, so a lot has probably changed.
I still use Hotjar for certain features that VWO offers, but which I think function better in Hotjar. I for instance prefer the Hotjar heatmaps, because I can elect to ignore certain elements on the webpage, such as a cookie consent pop up. A simple difference which makes me like Hotjar recordings more is that I can view a recording and click next, rather than going back to the overview and selecting the next recording.
It is well suited for exactly what you would expect. To gain insight into what visitors are doing on your website and to improve your website based on these insights through testing. Given my experience so far, I would say it is less suited to deal with websites that use AJAX. The insight gathering tools are still valuable, but the testing tools quickly become complicated.