There are lots of alternative mail clients available in the App Store and Mail Pilot for iPhone and iPad is the newest addition. While the workflow concepts are simliar to that of apps like Mailbox, the overall interface and design is rather different. Not to mention, Mail Pilot supports an array of clients including both Gmail and iCloud.
I backed Mail Pilot in its infancy stages last summer when it started out as a Kickstarter project. It was a great idea and I'm excited to finally give it the review treatment and find out whether or not it really was worth the wait.