don't use the gmail app is because I'm left handed and I like the button placement on the standard mail app for my left thumb. I also like the display of incoming mail notifications better coming from the mail app. Your preference may be different.
However, I also have the "Mailbox" app loaded for those times when I want better functionality to (1) attach a Dropbox file to email, or (2) search archived emails.
Seems like many people prefer the gmail app, but it just doesn't feel right to me for some reason.