I was able to hop on the new Mach 6 for a day at Outberbike this fall. Having had the first iteration of the Mach 6 3 years ago, it was nice to compare. The new bike is certainly longer and lower. I rode a small for both, but the new bike had a 40mm stem and felt good, whereas the first mach 6 had a 70mm stem and still left me pretty upright. I'm 5'8", and have always been able to ride either a small or medium for most frames. I rode the medium Mach 5.5 comfortably and would imagine the Mach 6 would have been fine as well, but I wan't uncomfortable riding the size small for 20+miles. The bike is really at home on wide open, fast, rocky descents, where you can stay off the brakes and hold on. On tighter technical climbs, especially switchbacks, the bike did require more steering input, and I can only imagine that would have been more noticeable on the longer medium vs the small. While more at home when the trails points downhill, the Mach 6 still pedals with the efficiency accustomed to the DW linkage.
Before the Mach 6 was released, I thought it had been replaced with the new Mach 5.5, and on paper the bikes look very similar, but those small differences do come to life on the trail. I would give the nod to the Mach 6 if you're riding big chunky terrain and are looking for speed on the descent above all else. If you're planning a big day in the saddle with as much up as down, the Mach 5.5 is probably your huckleberry. In any case, you can't go wrong with either of the new offerings from Pivot.
Please don't hesitate to contact me directly if you have nay questions on the Mach 6, or any of the bikes we offer