I just enabled this on my personal account, and my first impression is that the categorization is very useful. Sure, it would be nice to have more customization

I just enabled this on my personal account, and my first impression is that the categorization is very useful. Sure, it would be nice to have more customization available, but it looks like you can train it to some degree to tailor it somewhat. My main complaint is that it adds yet another set of controls on a cluttered and unintuitive interface that has already been changing recently. The tabs go away when you're looking at an individual email, so you can't go straight from a message in one folder to another category. As far as I can tell, when looking at a message and a new one comes in, you can't tell which category it is in. You only see that is there via an "Inbox (1)" in the sidebar. All in all, I think this is useful, but the overall Gmail UI is still disappointing, and getting busier and less intuitive instead of more helpful.