I have, but I'm not craving extra integration, just a more Cocoa-y chrome that loads faster. Mailplane still relies on loading Gmail's HTML/JS/CSS to display itself; it's no better in that regard than creating a Fluid.app called Gmail (which is what I've done for now.)

Right now I can click on my Gmail bookmark in an already-open Safari session, then open the Mail app on my iPhone, read and archive three messages and close it before Gmail is even finished loading on my computer. Mail.app, on the other hand, has a ridiculously-slow IMAP sync every time I open it, wants to keep a local index of every message (9/10ths of my used Gmail quota is in the index, since I always send files out-of-band) instead of using Gmail's search API, has no conception of labels, and does its own horrible local spam-filtering, and no conversation view.

So, to reiterate: why can't there be a native OSX mail client that uses Gmail's API, and exposes the UI affordances appropriate to that API, while being made completely of regular Obj-C/Cocoa, and not loading a Webkit component unless it has to render an HTML message?