Are all 39 tools actually useful, or is it padded with junk?

The set focuses on common sizes and drives for bolt-down work — no novelty pieces. You'll recognize every tool in there. That said, verify the specific tool list against your regular jobs before ordering.

Can I add my own tools to this case?

Yes. The case is designed for storage and transport, and you can add or swap tools as needed. Just don't overstuff it or the lid won't close properly.

Will this case hold up if I throw it in the trunk?

Hard panel construction is built for that kind of use. Expect normal wear over time, but it's made to handle transport between cars. The latch design keeps it closed under movement.