I’ve been using Mailplane for years. Kiwi is every bit as functional, and it provides a better “out of the browser” feel. Other than that, it is a high-fidelity presentation of the Gmail interface, except that you can use multiple Gmail accounts simultaneously. I have five accounts that I monitor, three of which I use heavily. Kiwi is up to the task.
Kiwi has a few rough edges. First, printing of e-mail isn’t quit working at the level of Mailplane, but Mailplane was never real good at it either. Printing from a Google preview presentation for an attachment doesn’t work in either product. In Kiwi, formatting of some mail with with embedded images can get stuck. It isn’t a common issue. Inbox doesn’t work in Kiwi. In Mailplane, it worked before it was supported. This too will pass.
In my short experience, Kiwi is on a fast release cycle as minor issues are addressed. This is great. But minor issues remain. For example, I’ve seen a message window not render formatted text entry correctly (e.g., underlined text disappeared from the screen). A refresh of the window fixed the problem (the message was correct), but the effect is wierd. Also minor.
This would be a five-star product except for the occasional glitch. The glitches don’t hurt the functionality because Gmail itself protects your work, but these sort of things were sorted out in Mailplane a long time ago.
Kiwi is faster, cleaner, and more integrated with MacOS compared to Mailplane. Both apps can only approach the speed and functionality of Gmail in Chrome, until there is nothing more to be done. Mailplane is mostly there. Kiwi is almost, and still improving.