I agree with trelawrence. Only reason to have it is to access old emails. You can not open or send attchments through the app so it's not really the most useful of the two options. I have it but don't even stay logged into it.
Posted from my CrackBerry at wapforums.crackberry.com
I use the Gmail app to access all my Gmail from even prior to having my BB. I don't delete emails in Gmail, so I have emails that date back a while. Thus, with the Gmail app, I have access to every email ever written to that address, which is extremely helpful. I keep a minimum of emails on my Berry itself.
That is one practical use. I don't use it every day, but I find it to be a must-have for me...
