You Can Downgrade From Android 6.0
If you get Marshmallow on board your device and you don't like the features or the performance, you should be able to downgrade.
We've put together some instructions that detail the process. You'll just need to substitute in the correct files. It's something to keep in mind as we inch closer to the OTAs. Maybe you'll like Lollipop better than Marshmallow, who knows.