Is this battery worth it over the larger Kobalt 24V packs?

It depends on the tool. For drills and impact drivers used in short bursts, the light weight is a real benefit. For saws, sanders or long jobs, a 4.0 Ah or 5.0 Ah pack runs much longer between charges.

Which tools and chargers does it fit?

It is part of the Kobalt 24V Max system, so it works with Kobalt 24V Max tools and chargers. It will not fit other brands' tools or older Kobalt 18V or 20V lines.

How long does a 2.0 Ah pack last?

Runtime depends on the tool. Expect a few dozen to a few hundred short driving tasks in a drill, and much less in high-draw tools like circular saws. Keeping a second pack charged is the practical fix.

Does it lose charge when stored?

Lithium-ion packs self-discharge slowly, so a charged pack kept in a dry place holds most of its charge for weeks. Recharge it before a big job if it has sat for months.

Can I leave it on the charger?

Kobalt chargers are designed to maintain packs, but for longest life it is better to take the pack off once it is full and store it away from heat and freezing cold.