To be honest, I don't get excited about email clients. I don't even like email, truth be told, but I like email a whole lot more than I like talking on the phone. I have multiple Gmail accounts for my various websites. Having to use Gmail in a web browser is painful, sloppy, and cumbersome. What I like most about Shift is the ability to just hide the window when I don't want email in my face, but I can easily switch to it when needed.
Switching between accounts is super easy and nicely organized (tabs). As far as the Gmail UI, once you are in the mail client it is identical. My guess is that Shift is built using the Node-Webkit framework so it is technically just a web browser with some nice bells and whistles in an enclosed environment. But that is not only fine, it's perfect. Why reinvent the wheel?
Shift is basically a much better UX wrapper for Gmail than Google currently makes available. It is such a simple idea but it is excellently implemented.
After using the free version for a bit I upgraded to Pro to help support quality work and for a few extra benefits & features.
Money well spent, IMHO.
I've used this product on and off for a year. Apart from the constant bugs, the overall experience of the product is pretty decent. However, they lost me when they added that awful " Sent with shift " signature in my signature with no chance of removing it.