I'll indulge in a little thread necromancy and add my 2 cents worth here.
I have an ACE 250. I also have a Bounty Hunter Tracker IV. I can say, without reservation, that in the extremely high mineralization of the soil where I live the 250 is the next thing to useless. I have to turn the sensitivity down so far that the depth is nearly surface level. The hot rock issue is no joke; I have many of them here, and the reaction of the ACE is nothing short of catastrophic. It takes several seconds for it to recover from hitting one.
The Bounty Hunter will pick up targets the ACE, because of the sensitivity level I need to run it at, misses completely, and ignores hot rocks that send the ACE bonkers.
The hell of it is, I like the ACE; I'm very frustrated that I can't use it! I'm even more frustrated that my $60 ebay Tracker IV (OK,Pioneer 101) outperforms my $200 ACE (in my soil, here in NW Arkansas, in my hands, having made the adjustments I had to to keep it in some semblance of stable. Your mileage may, and probably will, vary according to where you live.)
If anybody doubts my word about hot rocks, and owns an ACE250 and wants to try one out, I'll mail you one from my front yard.