Does 'all but useless' mean useless or useful?

all but phrase 1 Very nearly. ‘the subject was all but forgotten’ - ODO

The straightforward sense is that the reviewer was claiming that the product was nearly useless. This is consistent with the review complaining about the product.

Oddly enough, if you stress the word but loudly enough, the sense flips to a defense against the product's uselessness.

Consider:

• Reviewer: This microphone is all but useless. It hisses and spits, and barely transmits a word I speak into it.

• Manufacturer: On the contrary, it is all but useless. Obviously, you haven't paired it with the solid gold contacts it was designed for.

The "all" in "all but" shouldn't be taken on its own as meaning "everything", but together as a "just short of" or "almost". I can't quite come up with a good reason why this is though.

"All but useless" is saying "it is functional, it can be used, but it is so poor you'll barely notice it if it wasn't"

There are plenty of whiners out there, who throw in exageration, expletives, and colourful witicisms, but one who uses an "all but useless" is saying "hey I can be objective, I admit it works, BUT...." It lends a measure of credibility that can actually press a point stronger than just decrying the product.