I have recently demoed the Liquidmetal Prestige mid and I own a PC600. Here's my opinion:
PC600: The racket flexes nicely and feels like everything is flexing uniformly on contact. It has a buttery feel, and it seems as if the harder you hit the ball the better it feels (assuming you hit the sweet spot). The sweet spot on this racket is smaller. It seems as if the sweet spot is right in the middle. Hitting the lower part of the string bed feels fine, and hitting the top of the string bed is a little jarring (but not bad). This is due to the racquet's balance. The racquet offers some nice pop when swinging hard, but not much when swinging slowly. Overall the racquet has a classic, uniform, soft feel.
LM Prestige: The flex on this racquet is all messed up. The head of the racquet flexes all over the place and feels weird on contact. However, I think Head did this intentionally. The lm prestige is much more forgiving than the PC600, and I think it is due to all of the flex in the head. This racquet offers noticeably more power than the PC600. The power level seems more unpredictable than the PC600 though. The PC600 has a uniform power level, while only generating a tad bit more power when hitting the sweet spot. On the other hand, the lm prestige is all over the place. If you hit the sweet spot the ball has some nice pop. If you hit the lower string bed, there's not much power, and hitting the top of the string bed results in a much slower shot. The sweet spot on this racquet is notably larger. Not much larger, but the increase in size would probably result in more balls hitting the sweet spot. This racquets feels more flexible and softer than the PC600 if you hit the sweet spot, but if you don’t then it’s pretty stiff.
In the end, it depends on what you want. The lm prestige is more forgiving and offers more pop. The PC600 feels great when you strike the ball correctly, and feels nice even if you mishit. The reason why many people hate the lm prestige is because it feels completely different form the classic, uniform feel of the PC600. I personally prefer the PC600 because it feels better when I play with it. However, I did notice that I hit the ball harder and hit more balls in with the lm prestige. In my opinion, they’re two completely different racquets. These are my opinions, but I think it’s a pretty good analysis.