Still have a couple original Ubuntu CDs floating around the house after 2 subsequent moves.

I ran it for a while on my ultra 24 until I just installed linux on it. But it was interesting dipping my toe in the solaris world

What does your setup look like? What kinds of workloads are you using it for?

I submit that the problem was Solaris 9 just being massively overtaken by desktop Linux in every way - easier to install, faster, simpler to patch, better GUI, more software available. Solaris 10 was too little too late; it already had to compete with mature Linux offerings that had a greater mindshare, and the excellent features like Zones were too far ahead of their time (and lacked the centralized store of downloadable Zone-prepackaged apps that is really the reason for Docker's success).

Purity doesn't matter in practice - especially in a world where OS installations are increasingly ephemeral and ideally immutable.

I always hear DTrace is awesome, but have never used it. And seem to have gotten by just fine... what am I actually missing?

ZFS, on the other hand, was so good that it has outlived Solaris itself and is at the core of e.g. TrueNAS as a commercial product.

Do you have any recommended references to understand that concept and how it plays out in Solaris?

I find myself in the middle of what I imagine are similar design challenges in software packaging, storage, integration, and deployment.