As many commenters have stated, support is generally excellent. However, please note: I had an issue in which a WPMU DEV plugin that is vital for my business had stopped working properly.
If you look at the comments on the thread I created in their forum (https://premium.wpmudev.org/forums/topic/unable-to-edit-appointment-time/#post-*******), you'll see that this issue affected not just me but likely all users of the plugin, which makes the issue even more relevant.
It simply took them from May to December of 2018 to come up with a workaround. To me, this meant seven months without an essential plugin.
And they ignored when I asked to not be charged their monthly fee while they came up with a solution. Please note that I only use this one plugin from WPMU DEV. So all those seven months i kept paying monthly for a plugin that had been rendered useless by malfunctioning.
I give WPMU DEV five stars because, in all other respects, I agree with what has been said about them. And I am inclined to believe that what happened to me was not typical of their customer's experience. In all other instances, through the several years I've been signing up to them, I had really excellent support and experience.
So I would still recommend WPMU DEV to my friends, and would chose any of their plugins if I ever need them. Just keep in mind that nobody is perfect, not even ourselves.