Does the M18 come with a battery?

No. It is a bare tool, so no battery or charger is included. You need an M18 pack and charger, or a kit version, to run it.

Will my existing M18 batteries fit?

Yes. Any genuine Milwaukee M18 18-volt Li-ion pack fits, and third-party packs made to the M18 mount generally do too. Capacity and age of the pack affect runtime and power.

Is it worth it if I own no M18 batteries?

Only if you plan to buy into the platform. One pack and charger can then serve many tools, which is where the value comes from. For a single tool, a kit with battery and charger is usually the simpler buy.

How does M18 compare with Milwaukee's M12 line?

M18 tools are larger and more powerful, suited to heavy drilling, cutting and sustained work. M12 tools are lighter and more compact for overhead or tight-space jobs. Pick the platform that matches your work.

What should I check before buying a bare tool?

Confirm the tool body is the model you want, since M18 covers many different tools. Check that you have at least one charged 18-volt pack and a compatible charger.