When setting up iCloud in my iOS 5 iPhone 4, there is a new @me.com address created (Settings > iCloud > Account > Advanced > Mail
Is this new @me.com a full feature mailbox?
Yes - iCloud includes full mail service - webmail, IMAP and iOS for free and not counting against your free 5 Gb of cloud storage quota.
http://www.apple.com/icloud/ mentions that iCloud is free with iOS 5. Enjoy!
I've been using my mac.com and me.com address as my main address since iTools day (when the predecessor to Mac.com was free). Never had any reason to use anything else. I have gmail, but only use it for business emails mostly.
iCloud has a web interface for it, and the Mail.app on iDevices and Macs all use it for their main account. Works well most of the time, though there were a few hiccups tonight with the server authenticating my password. But that seems to have passed.
@bmike is correct that iCloud is a full mail service. But, it counts against your free 5 GB of cloud storage:
Free gets you a lot. When you sign up for iCloud, you automatically get 5GB of free storage. And that’s plenty of room, because of the way iCloud stores your content. Your purchased music, movies, apps, books, and TV shows — as well as your Photo Stream — don’t count against your free storage. That 5GB goes a long way for your mail, documents, account information, settings, and other app data. And if you need more storage, you can easily purchase an upgrade right from your device.