It appears like Developer Previews are going to be the norm for Google going forward. Last year, we saw the launch of the Android L Developer Preview, which we all knew to become Android 5.0 Lollipop. The announcement from Google I/O 2014 seemed strange at the time, but it makes a lot of sense. A Developer Preview gives developers the chance to get familiar with the new APIs before a proper launch, rather than just dumping them all into developers' hands just before the new source code hits and updates start to arrive. Plus, this also allows features to permeate throughout the community, rather than being a complete culture shock when they launch.
This year, we saw the launch of the Android M Developer Preview, which probably won't be called Milkshake when it launches later in the year. Unlike last year's Android L preview, which laid dormant for quite some time before Lollipop launched, it looks like the M Preview will be updated regularly as time ticks down to the full launch in the Fall. The M Preview page by Google outlines a timeline that kicks off in May with Preview 1, and then sees monthly releases for June and July with a release in "Q3". This could be a sign that Google is looking to add more APIs to the M release, or just keep Developers up-to-date, rather than leaving them in the dark as many felt last year during the L Preview.
Android M is quite clearly about added polish, and keeping the Android platform up-to-date and fresh. Building from last year's Android 5.0 Lollipop release, Android M is pushing battery life even further with Dozing and adding better security with fingerprint support and of course, Android Pay is a major new feature as well. if you haven't spent time with the M Developer Preview just yet, you can download it here and while it's not for those that aren't willing to get their hands a little dirty it offers a neat glimpse into the future of Android, due out this Fall.