I had an M6 for awhile. My dirt in Colorado is mineralized and the M6 had trouble with depth and solid IDs past 4" no matter what coil I used. Otherwise, it was a very dependable, solid detector and I had no complaints.
My prospecting/water detecting addictions meant I wanted to try an Equinox. The Equinox is an outstanding, somewhat complicated detector that took some getting used to, multiple manual reads, and patiently using Park 1 default for a month with a lowered sensitivity level because of the mineralization here. I now have over a thousand hours on it and I know what I know about it very well. It truly is a great coin/jewelry detector, can go plenty deep in mineralization, works well at salt water beaches and has found gold nuggets for me. I haven't done much relic hunting with it so far. There is still plenty on it that I have not explored yet.
I am not going to say that it made all of my other detectors obsolete. It did become pretty obvious that three of my detectors were not going to be needed anymore so I sold the M6, a Fisher F19 and an X-Terra 705. I kept my gold detectors and recently picked up an XP ORX which is pretty awesome too.
Jeff