Akuhn,
I am not sure about how the other companies deal with this issue, but we tell the customer that the technician look at it and verify that it will work properly with the phone. Some headsets, such as Plantronics, use the Handset cord and in those cases there are very few issues as it is tapped inline with the handset; old school but it works. It is usually when you are using the Headset port on the back of the phone, which you should use if you can, that some headsets do not work "properly". As far as the audio of the built-in headset/handsets that Mitel manufactures I have not had any issues with it in some time now, apart from the occasional bad part which happens with all electronics. As for the Plantronics Headset with a lifter be prepared to take a little time setting them up properly, but once you get one set on the right spot then it is a cookie cutter process to get the rest of them done. I suggest using the phone in its laid back, lower profile, position as there are less issues with the handset failing out once it goes off-hook due to the narrow rounded base at the bottom.
Thanks,
TE