I also bought one of these from Aliexpress for about $22 delivered. Not having ever used an AirTag I can't compare to that, but it certainly works as expected. The plastic feels a little cheap even with low expectations, but realistically who cares on a device like this, so long as it's sturdy, which I think it is.
I think NZ Customs may have removed the coin battery — it had an empty hole in the internal packaging where it should have been. Seems weird though?
More than happy to run some kind of test if anyone's curious and wants to tell me what they want to know :)