OK, so I've just came back from getting my wife's KIA's transmission flushed, which we do every 50,000kms.
I asked him about my E46, and he said he knew them very well but they don't see as many these days. He said all the "power flush buggering the gearbox" is bullshit, but not for the reason everyone thinks. He said they don't "power flush" them at high pressure at all, it's just a transfer pump that hooks up to the return line from the heat exchanger. He said that it puts hardly any pressure at all into the line, it just delivers it back to the box and the internal pump pressurizes it there at the same pressure it always runs. He said he hears that story all the time, and it's more likely they came down for a flush because they suspected their box was buggered already.
He did say that if there was massive buildup on the magnets there's no point, as it's already on it's way to failing. He also said the BMW pump flows pretty quick, so you need a pump that will keep up with it, or be ready to shut it down if the open hose you're running to the waste oil bucket starts bubbling. He said if that happens, shut the car down and wait for oil to come out of the fill plug (that you've left open) then start the car again.
He did mention "for fucks sake don't hook the compressed air up to it", which may very well cause the things mentioned, but he said flowing clean fluid in this way can only be good and "people just say these things because they read it on the internet". His words, not mine.
He quoted me 550AUD for a full flush with filter etc, which is too much for me for that car. When I said that he said just get an ebay transfer pump, pull the quick fit hose off the inlet of the heat exchanger and gently clamp some soft hose onto the pipe and pump it in there for a full flush. He also said the box should be fine at my miles unless it's been driven hard or badly, but it's definitely time to service it and keep on servicing it.
I've got no reason to doubt him. We've used him for a couple of years and his business is always very busy. He's a good guy too, and seemed to know the E46 very well.