How accurate is a handheld luggage scale like this?

Handheld strap scales are generally close enough to flag an overweight bag, but they are not laboratory instruments. Weigh the bag two or three times and see whether the reading holds steady before you trust it.

What weight can it handle?

The maximum capacity is not stated. Check the scale's own labeling before weighing a heavy checked bag, and support the bag from below on the first lift.

Does it work for both carry-on and checked bags?

Yes. The strap and hook attach to a suitcase handle, so you can weigh any bag you can lift. It is most useful for checked bags near an airline's limit.

How do I weigh a bag with it?

Attach the strap or hook to the bag's handle, lift until the bag hangs free, and hold still while the display settles. Weighing at the hotel before the airport gives you time to repack.

The scale with its strap and hook attachment. Battery details and any pouch are not stated, so confirm the contents before you rely on it for a trip.

Is it worth it compared with a built-in airport scale?

Airport scales are free but only tell you the problem at check-in. A pocket scale lets you fix an overweight bag at the hotel, which is the reason to carry one.