Just as a datapoint youtube or vlc seem to work well enough. The sluggishness is definitely noticeable though. As a developer it encourages me to optimize the display performance of my web applications. If it performs ok on Qubes it's liquid smooth on my 3+ year old android phone. Take that as you will...

> internet/firewall/audio would randomly stop working when restarting VMs

I started with 4.X and I haven't noticed this. I never actually used 3.X so I can't say if it's something that was fixed or never present on my hw.

> provides a functional but very tightly-knit product that cannot be easily modified to suit your own needs.

This is a fair assessment. But I've come to realize I need the isolation Qubes provides more than I thought I would. No advanced threats or anything. Just the ability to put email, chat, password manager, and work in separate reproducible environments is really cool. Clicking a url on a chat won't open in the browser I'm signed into google or in my work browser. If you have the need for gpg or tor they offer solutions with a high degree of isolation, though I haven't looked into them much.