I've been using Gmail as my primary personal email since it was Beta. I am totally in love with Gmail. It is fast, secure, and with Chrome being my main browser, the experience has been beautiful. Gmail is even more attractive on Android devices. Unfortunately I can't say the same on iOS devices. I can work with Gmail app on iPhone, but lately I have been using an iPad Pro a lot. The Gmail app experience is horrible. I've read about Spark. The more I read about Spark the more I felt drawn to it. I saw couple of YouTube Reviews. The app is just beautiful.
Spark allows connecting several email services, just like most email clients these days. I've connected so far my Gmail, Hotmail, and Yahoo emails to it. The "Smart Inbox" segregates new/unread from the read ones. All email messages from all services appear in inbox. A feature available on Android but missing from Gmail's app on iOS. Where Spark falls behind (in my opinion) is with the labels feature. I am hooked and addicted to Gmail's labels, but can't seem to find a way with Spark to assign those labels. The main features where Spark excels are the gestures and settings with plenty of useful features, like Pin, Snooze, Save as PDF, Archive, Mark as Spam, etc.
What I couldn't figure out yet is the "Connected Services" part. I connected my Dropbox and Google Drive to allow file attachments. But I don't get the Evernote and Pocket connections since iOS allows sharing to those apps directly.