Unfortunately this does not work for accounts of the type "GMail", as far as I can see. (Maybe you could configure your gmail account as an IMAP account, but then you lose a few gmail-specific features)
That is correct; if I use the 'Google Mail' setup and add an additional mail address, the IMAP login no longer works since the mail addresses as a whole are taken as username. In the default IMAP setup, sender mail addresses and the user name are separated.
Are there any other Gmail-specific features except for 'Archive'?
My main annoyance with Gmail and local mail clients – including Mail.app for iOS and OS X – is that those clients show mails at least twice due to Gmail's label structure if you enable all labels. I am therefore glad that the new Gmail app for iOS is much more usable. On the Mac, I use Mailplane and Mail.app only for backup and offline usage purposes.
It seems like "GMAIL"-account types have real push? Or at least I'm seemingly getting new mail much quicker than I ever used to get on IMAP?
Plus it integrates much more smoothly into the calendar app as well. Better than fiddling around with webcal links and the very well hidden multi-calendar selector at https://www.google.com/calendar/iphoneselect )
Never had a problem with duplicate mails, in fact I'm having a problem with de-duplicated mails (When I send a mail to a mailinglist, I never receive a copy back that ensures proper list delivery, because it gets de-duplicated to the copy in Sent mail)
