I’m sure some you won’t like comparing GMail mobile application to Microsoft Outlook, but the truth is that Outlook is quite good mailing application. GMail mobile is now much closer to this definition. New GMail 2.0 released last week is no longer only webmail access to your single GMail account. It offers: support of multiple e-mail accounts (both GMail and Google Apps), basic offline access to you latest mails (those displayed in on the phone screen). It allows composing new e-mails offline and sending them automatically in a background. You can create and use defined mobile e-mail drafts as well. In general the application for Java supporting phones and Blackberry devices works more stable and gives more user friendly keyboard shortcuts.
I personally have it on my E71 as a search engine and backup of my main e-mail account, but now with multiple accounts support I think I’ll use it more frequent. Please check interesting E71fanatics.com post about new GMail, or view official GMail blog movie about it. The application itself is accessible under m.google.com/mail supporting 35 languages.