I'm only a few days into using Inbox, but so far it's looking great. The interface is well designed and clean, and it's fairly straightforward to use (support have also been very responsive). I've mostly used Mailchimp in the past, but the biggest advantage I can see Inbox has over their competitors is that you can manage mailing lists for multiple brands within the one account and send from multiple different email domains (which makes it much quicker and easier than constantly having to log in and out of different accounts).
Really looking forward to seeing the automation functionality get implemented too!