Gmail user who recently spent five weeks in Brazil, where "any form of permanent [Internet] connection was far from possible." In other words, he quickly grew frustrated with Gmail because of his limited connectivity.
After all, Google's mail service lives in your Web browser, and when you can't get online, you can't access your messages (new or old). This is true of other Web-based mail services as well, like Hotmail and Yahoo, but Google recently unveiled a new option for offline mail access.
Known as Gmail Offline, this browser plug-in allows users to read, respond to, and search mail even when there's no Internet connection. It has a nice-looking, simplified interface that's similar to what you typically see in Gmail--but with fewer options.
Unfortunately, for the moment Gmail Offline is available only for Google Chrome. On the plus side, similar plug-ins are forthcoming for Google Calendar and Google Docs.
There is, of course, another option, one that can solve the offline problem for not just Gmail, but also Hotmail and Yahoo Mail Plus: an e-mail client like Windows Live Mail 2011 or Mozilla Thunderbird 6. Programs like these can be configured to download e-mail and store it locally, thus allowing you to read, respond, and search--just like with Gmail Offline. Just make sure you stick with a POP3 configuration rather than IMAP, as the latter requires a live Internet connection for many functions.