I owned a Quest Q40 for awhile and still own some of Quest’s wireless headphones and transmitters. The Q40 was a perfectly good detector that acted much like a Teknetics T2 (see the post above by Dave E) which means that where I detect which is mostly high mineralization, the target IDs were unreliable on low and mid conductors on anything deeper than 3” just like on the T2, F75 and all other single frequency detectors. There is nothing magical about the Q20 and Q30’s target ID capabilities either. I have used a Q30 and it will lose target ID accuracy in high mineralization just like my Q40 did...........everything got up-averaged into the silver range the deeper the target was or had no ID.
My only problem with owning a Quest detector at the moment is where do I send it for warranty repair? Quest USA is still (at least the last time I checked) in litigation with First Texas Products since they broke the rules of their first settlement mentioned by Dave E above. They are not allowed to have any kind of business offices or manufacturing in the USA. Whether they have contracted with someone to do their USA repairs, I don’t know. If I were to consider buying one of the detectors new, I would first find out if and who is handling their warranty service in the USA.
The fact that Quest/Deteknix seems to have survived two lawsuits with First Texas and the current pandemic, speaks well of their determination to produce good quality products (hopefully not infringing on copyrights) that must be selling well somewhere.