my 16 yo son has a trek 6500, which he likes a great deal,
we got it a couple of seasons ago.
purchased it at the end of the year from a mountain bike shop who had it as a summer rental.
the cost was about 1/3 of new.
changed the tires to Schwalbe Big Apples,
swapped the flat handlebars to origin 8 space bars, that are angled more like a cruiser bar;
they are almost identical to the on one mary bar, half the cost.
he tried a bunch of seats and ended up with a Brooks B-68. Also installed Kool stop brake pads.
most important thing was working to make sure the bike fit well,
he was inpatient with me about this at first. finally as he rode more he appreciated the subtle changes
in seat position or why we needed to swap our two stems before one worked ok. we did a 680 km tour
last summer, planning another this summer, plus other rides.
we have been to a few bike shops recently, see him glancing at other bikes- when I suggest trying another- he always says he likes "his bike". fine with me.