Verdict. The second thing to buy after a scale. Doneness is a temperature, and every other cue — colour, firmness, time — is an unreliable proxy for it.

“Roast for 20 minutes per 500 g” assumes a specific starting temperature, a specific oven, and a specific shape of joint. Change any of them and the time is wrong. Internal temperature does not care about any of that — it is the thing you actually mean when you say “done”.

What to look for

Read time under 4 seconds. This is the whole product. A slow thermometer gets left in the drawer.
A thin tip. Thick probes leave visible holes and read the temperature of a wider region, which matters in a thin fillet.
Water resistance. It will be used with wet hands, and it will be rinsed.
A fold-out probe with an auto-on hinge. Small convenience, used daily.

Calibrate it

Ice water should read 0 °C, boiling water 100 °C (a little lower at altitude). Most units have a calibration screw or a hold-two-buttons routine. Check when new and after any drop. A thermometer that is confidently 4 °C wrong is worse than no thermometer.

Where to put it

The thickest part, away from bone and away from the pan surface, and push through to the far side then withdraw slowly while watching the reading — the lowest number you see is the centre. Reading the first number that appears is the most common error, and it usually reads high.

Leave-in probes are a different tool

A cabled probe you leave in during a long cook answers a different question: “how is it progressing” rather than “is it done”. If you braise or barbecue, own both. If you roast a chicken once a week, the instant-read alone is fine.