[Updated at Wednesday evening with new information from Google.]
The main difference is speed. Regular Gmail is generally fairly quick, but you can still find yourself waiting at times for it to check in with Google’s servers. In offline mode or the very cool Flaky Connection Mode, everything — opening messages, searching for information, labeling missives — happens almost instantly, since all the data is local.
The tradeoff is that you don’t have access to all of your mail. For my account, the system synced up around 6500 messages, about two months worth of email. (It’s not clear whether Gmail limits the cache by the number of messages or the length of time since they were sent. I’ve got a question in to Google to find out.)
UPDATE: A Google spokesman sent me an answer from their support forums that’s both more complicated and interesting than I expected. Google says, in essence, that it downloads your 10,000 most recent messages (they estimate that will cover several years for average users). But they also say that they identify your most important email threads and sync those. That’s a fascinating idea that’s not terribly well explained. Here’s the exact quote:
“We try to download your most recent conversations along with any conversations that seem to be important (regardless of their age). We also try not to dowload uninteresting conversations. This process is done heuristically and as with any heuristic can and will miss things. We’ll continue to tune things up, but more importantly, we’ll eventually provide a UI that will allow you to change the settings. Here’s a sketch of how these messages are selected:
Synchronization is based on the date of conversations. The system estimates a period of time to cover (at least 1 week in length) that results in approximately 10,000 messages being downloaded. For an average user, this means Gmail will end up downloading several years of mail.”
One mystery: Google’s offline settings reports that it will sync all my messages that have one of four labels. One of those labels is Starred, which makes sense. The other three categories seem to have been chosen at random. And I couldn’t find any way to change which labels get the full backup treatment.
UPDATE: Here’s Google’s explanation of that mystery:
“We’ll download any conversation marked with a label that contains less than 200 conversations, has at least one conversation that has been received in the last 30 days and also has at least one conversation that’s outside the estimated time period. For many users, this list of labels will include Starred and Drafts.
Finally, the system determines a list of labels to exclude conversations from being downloaded. For example, Trash and Spam are always in this list, along with any label that contains mostly unread conversations (unread count greater than 99%). So, we won’t download a conversation if it contains only labels in this list. A typical Trashed message will not be downloaded, but a Trashed message that contains the label “alpha” will.”
You turn on offline capability through Gmail Labs. If you haven’t already, you’ll need to install Google Gears, the background system that enables offline capability in services like Google Documents and Zoho Mail. Once you okay Gmail using Gears, it’ll start downloading messages. On the EV-DO connection I use during my commute (the very definition of a flaky connection), Gmail synched about 150 messages a minute. Once I got on my home broadband connection, things went much faster.
Working with Gmail in offline mode is almost indistinguishable from working online. When you compose a message and hit Send, Gmail will put the message in your outbox and send it the next time it has a connection. When you search for mail, Gmail will warn you if you’re seeing only a selection of the mail that fits your search criteria and let you know that you can find more when you have a Net connection.
You can’t edit your contacts while you’re offline, but the system will still autocomplete addresses for people in your contact list. One surprise is that Tasks aren’t available in offline mode – that seems like it would be a fairly simple thing to sync and something that most users would want access to when they’re offline.
Flaky Connection Mode seems like a real winner for anyone whose connection is at all unreliable or just slow. The system works with the local cache of data, so everything happens very fast. But Gmail still syncs with its servers whenever your connection allows, so you still get new mail and messages you compose are sent as soon as possible. As long as you don’t need access to messages more than a few months old, it’s the best of both worlds.