Depends on how your battery has been testing you. I've been running it, and Android M seems to be pretty power efficient at rest. However, it's a developer preview and can go a bit crazy at times. I've had processes go wild and eat over 50% of the battery in an hour. If you're looking for a major difference or boost at this point, I don't think the preview will be that satisfying.
Overall, it's been a good experience, and I really haven't thought about going back to 5.1.1. WiFi connectivity for me has been somewhat spotty and never auto-connects at this point. Hoping that the next preview solves some of the minor issues.