I tried it for 15 minutes and went back to the old design.
It feels fresh on a UI level, but in the end usability is what really matters for a tool. In general I think UX is worse now.
Text is harder to read with the new font. Also there is just too much white, everything is floating with no visual structure, no contrast. Icons are smaller. Animations aren't smooth in my top of the line 5K iMac. Etc.
This new Gmail doesn't have the quality and polish that we've come to expect from Google's web apps.