Upgrading to M worth it if rooted?

Just wondering, my Nexus 6 has recently been excruciatingly sluggish and I wonder if M could save it from the trash.

On the other hand I'm all rooted and don't remember how to do it and not sure I want to spend time reading up again and going through the hassle of maybe loosing data, etc.

Is Android M worth it?

IMO doze alone is worth it.

How would you suggest I upgrade to *rooted* M?

Flash the "M" image (you can skip userdata if you are on the stock Lollipop ROM), and then, reroot (rooting 6.0 requires a custom kernel at the moment)

Having said that, the N6 shouldn't be sluggish on Lollipop in the first place. Try clearing the cache partition, and rebooting to see if that helps with the sluggishness.