I use Gmail for email but iCloud for all other data (calendar, contacts, notes, reminders, etc.).
Main reason - where I work the wifi firewall blocks all POP and IMAP email but allows MS Exchange. And since Gmail can be configured on the iPhone as Exchange, this works out perfectly. iCloud (and .Mac and MobileMe before that) don't use those protocols to sync the rest of the data, so those go thru the hospital's wifi without trouble.
I've got 3 email accounts set up as Exchange - two Gmail and one enterprise Exchange account.
So, this way, I can get my email while on wifi at work while maintaining nice and easy contacts, calendar, etc., syncing with my iPhone, iPad and Macs.