I only started using Tree Style Tabs after it became a WebExtension. The userchrome.css stuff is still required, but it is documented and TST will link you to the doc when you first run it.
The usage experience is good for me. I had one weird bug where the TST panel would not correctly initialize in a new browser window maybe 10% of the time, but I cannot recall this issue happening in the last two weeks, so maybe that was fixed by an update.
The usage experience is good for me. I had one weird bug where the TST panel would not correctly initialize in a new browser window maybe 10% of the time, but I cannot recall this issue happening in the last two weeks, so maybe that was fixed by an update.