Certainly!! Gmail's offline version (Gmail-OV) can be considered as a real-time alternative to desktop mail clients. The offline version provides almost all functionality that you would look for in a Desktop Based email client.
Gmail can truly become an application rendered by Chrome. Here is how it can? When you double-click an application shortcut, the website opens in a special Google Chrome window that gives you as much working room as possible. It shows up on your computer taskbar like another program that's open, back, reload and forward buttons among others. Since the offline version is powered by Google Gears' local cache, you can actually download the files from the local cache. (Follow this link to know, how?). Now, say your Internet connection goes down, you can still continue to read your messages and reply them, so you don't have panic 😉 until the connection goes back up.
Even if you have a fast Internet connection, it's faster to load data from your computer than from Google's servers, all thanks to the "flaky connection mode" / "unstable connection mode" feature! Other features equip you with the ability to cache messages for more than one account and switch between them, and seamless change of online/offline status!
This Offline version has some drawbacks too, like its inability to add attachments to messages composed offline and the unavailability of the contact manager in offline mode. You can`t customize which messages are downloaded, in the offline mode either. Here is a link to all the features of the Offline version of GMail, from Google!