When Google made the move to Inbox, I switched because I wanted to be using their newest interface. Over time I came to absolutely hate it and that drove me away from Gmail.
My wife still uses it so I occasionally find myself in the Inbox app on our tablet. Just a few days ago, actually, I read something in her inbox and for the life of me I could not figure out to mark it as unread from the inbox list. Maybe I was just missing something obvious, but it was very frustrating to not have such a simple action easily available.
Gmail's interface is very limiting, actually. It's pretty good for a web app, and probably better than Outlook/Exchange, but when the competition is so poor it's easy to seem good.
Chiming in with a "both" - I tried switching away from Inbox for a bit but I love me them bundles. I don't want to switch between a bunch of folders, I just want mail in those folders to appear grouped in my inbox
Edit: and Snooze. I don't think I'll ever like using a client that can't snooze an email anymore
It works really well as a 'TODO' list for email, plus with niceties for travel and what not.
FastMail does this incredibly easy since you can set up an inbox to allow any alias, so you don't have to create them up front. I just type it in and whenever an email comes in it'll be routed properly. Gmail has this too I believe, but only with the + syntax (e.g. firstname.lastname@example.org) but unfortunately too many places have broken email validation routines that doesn't know how to deal with + signs.