Wow I just posted a very similar question! lol
Actually we are quite pleased with what the new UI offers overall. We are targeting it for a subset of our overall projects. Our "Traditional Waterfall" projects which are led by professional PM's from the PMO will still be managed within Classic. They have the option of utilizing the Conversations and Status dashboard pages in the new UI to help facilitate team collaboration. But they understand that all other management functions need to rely on classic at this time.
About 40% of our projects are not traditional waterfall, are not managed by a trained PM, and can easily make good use of the new UI for the full lifecycle management of their projects. Sure, there are a few features they will lose, for example Stage Promotion. But we asked ourselves whether that was actually adding value for these types of projects, and elected to live without it.
As stated in my parallel post, our only real gap is figuring out how to create Team Allocations while still "living" in the new UI - either by transposing ETCs to Allocations or by some other magic.