The Android N developer preview is available now OTA if you’re interested. It works on several Nexus devices and if you want you can simply download and upgrade OTA after signing up for the Android Beta Program.
It looks a little different, allows for split screen multitasking by long pressing the select button, notifications can be bundled, and it appears there’s a better dismiss all option for notifications (but I may be imagining that last one).
I don’t have a lot of playtime on it yet, but the split screen is actually quite useful for trying to look up something to correct your horribly racist friend.
On my Nexus 9 I simply signed up for the Android Beta program above, and a few minutes later an OTA notification was in my status bar. It downloaded, took about 15 minutes to install, and bam.
Being in the first 24 hours of using it I really can’t comment on anything other than that it’s working.
There is no codename listed for N, but it’s probably Nutella or NERDS given they’re both awesome.