The user can specify what to download via labels and for how long (what time period) - understanding that the more you download, the bigger the footprint on your local system.  Remember also that Google mail identifies each piece of mail with a label tag - a label tag can be inbox, starred, sent mail, and all variations of customized labels.  Typical settings are to download all items tagged with the label inbox and starred and then a time period of 2 months for all other tagged mail.  Anything archived, deleted, etc. will not be in the local system.  Additionally, the user can specify the threshold of file size for attachments to an e-mail - such as specifying that all attachments less than 5mb be synchronized on the local system.

Offline access works very well and allows the user to read and compose messages when not connected to the internet.  Upon connection, the local system synchronizes with the Google cloud by sending out composed mail and receiving incoming messages.  Anything archived, deleted, or tagged while offline then does the same to the data within the cloud upon synchronization.

To enable offline mail access the user simply selects the Settings link and finds the Offline tab - set the parameters as desired and save.  A little green dot next to the settings link will whirl around until synchronization is complete and then will display as a green check - that's the status that all is ok.

One last helpful hint - within the Settings --> Offline tab is a link to create a desktop shortcut when offline - this is extremely helpful and recommended as funny things happen with browsers when they're not connected to the internet.  By using this link, the browser opens up directly into offline mode.