CPT-20 : Decent budget option

Good:-

This is a cheap and cheerful A440Hz [not adjustable] tuner with proper bypass.

Nice and quick to react, and makes it clear when a string's in tune.

Copes with Drop A on guitar, and B on a 5 string bass if it's first in chain, and happier with bridge pickup signals.

You know when it's on as the indicators can be seen from the ISS. It's also got a positive footswitch.

Tiny footprint on a board.

Meh:-

Clearly made in the same factory as EastRock, Donner and others, as it's identical, down to the magnetic flappy box. It also has exactly the same flaw, a loose jack socket. The hole in the case has been cut slightly oversized, so the socket is loose, but the jack itself is a good fit. Mine's in a pedalboard, so accidental disconnection isn't a worry.

Only powered by the socket [due to it's nano size].

No way to adjust frequency from A440HZ.

TLDR:-

Cheap, cheerful, fast but built to a price. Worth that price.