I managed to get myself an Inbox by Gmail invite just to test it out. My overal impression is that this still seems like a very beta interface and in terms of functionality, I think I would prefer the old Gmail interface.
So short story, I've been using Gmail for a long time now. Was probably one of the first few to adopt it in my local country and basically, it's my default mail provider. I like gmail a lot simply because it has a lot of functions that are linked to it, from calendar to blogspot to google+ etc. The fact that it has 30GB of free space and a fairly secure network doesn't hurt either.
Anyway, enough of that boring backstory. On to the new interface by Gmail. This isn't the first overhaul of its GUI that Google has ever done but the last one was years ago.
First of all, let me just say that while I can understand where they're coming from in terms of the design, I feel that the new Inbox app for Android still lacks some functions. The first thing you'll be greeted with when you open the app is a main overview of your inbox, as well as headers with all the different new mails you've received in different folders. Last time the app just told you how many new mails there were but now they tell you the sender and sometimes show a mini-preview as well. This is good.
However, and this is a huge thing for me, they've gotten rid of swipe to delete. There isn't even a setting for it anymore, which is so weird. Instead, now when you swipe, you just remove the email from your main inbox view. It's still there in your folder, it's just not seen from your main inbox panel anymore. As someone who gets quite a few mails a day from various Kickstarters/shops/etc, there are quite a lot of mails that I end up wanting to delete because they're just generally for my own personal knowledge only and not something I need to keep around.
However, I will say I like the Pinning function, which I feel is really an overhaul of the Starred function in the old Gmail UI. This allows emails that you deem worthy or important enough to be pinned to the top of your inbox. You can just to disable this view by simply clicking the pin on the top right-hand corner of the internet.
However, in terms of functionality though, there is still a lot to be desired. At the moment, there seems to be ONE layout present for the new GUI and while it's a nice looking layout, I can't help but wish it was more compact or it had a way to present more information at once instead of having such large fonts or header bars and so on. Otoh, Google IS using the same layout for their new Calendar GUI and I think this would be something I feel would be more suited to a calendar app than a mail application. So that is something I'm excited about at least.