Let's get the most important information out of the way first -- it does, in fact, work. Turn off your WiFi or unplug your Ethernet cable and Gmail just keeps on keepin' on like it's nobody's business. You can keep reading your mail and any new messages you compose will be sent out the next time you connect. But the tale doesn't end here.
Once you've got that app conflict cleared up, the first thing you'll notice (besides the new, blue icon) is that Offline Google Mail looks almost exactly like the web app for tablets. And there's good reason for that as they're essentially the same thing. Google added a few niceties, including a couple of keyboard shortcuts and the ability to tack on attachments, but the basic design and feature set is largely based on the iPad HTML5 app that debuted last year. The inclusion of the J and K shortcuts to navigate between messages is greatly appreciated, but the fact that X (for select), R (for reply), F (for forward), C (for compose) and / (for search) aren't supported (to say nothing of Gmails other powerful keyboard commands) is an unfortunate oversight.
The UI and feature set definitely need some work. The two-pane layout is nice, but it's clearly not optimized for interaction using a keyboard and mouse and it feels somewhat restricting coming from the full-fledge Gmail page. That being said though, it works as advertised. All the changes we made, be it marking something as read, deleting a message or drafting a reply were automatically synced upon our next connection. We were even able to close all our tabs and Chrome while offline and reopen right where we left off without skipping a beat.