I have them. They offer very good handling with good ride quality (they actually have pretty soft sidewalls for comfort I assume, but like I said, they offer good handling). My previous cars include an '01 Z06 and '06 Evolution IX and they were both excellent handling cars. All of the turns I've taken quickly in the Z06 and the Evo, I've taken just as fast in the C32 with these tires. They offer decent wet traction and are speed rated to 186 mph (92Y). Are they the end-all of performance tire technology... nah. Straight line grip offers a little to be desired and they aren't cheap. Would I buy them again?... I already have.

Now, there are two different types offered on tirerack.com. One is the P Zero Rosso, and the other is the P Zero Ross (Mercedes). The difference is that the regular P Zero Rosso is "only" rated at 186 mph as the load rating is 88Y. The P Zero Rosso Mercedes has a load rating of (92Y), thus a speed rating of 186+. Notice the parenthesis around the load rating.

"Most recently, when the Y-speed rating indicated in a service description is enclosed in parentheses, such as 285/35ZR19 (99Y), the top speed of the tire has been tested in excess of 186 mph, 300 km/h indicated by the service description as shown below:

285/35ZR19 99Y 186 mph, 300 km/h
285/35ZR19 (99Y) in excess of 186 mph, 300 km/h

As vehicles have increased their top speeds into Autobahn-only ranges, the tire speed ratings have evolved to better identify the tires capability, allowing drivers to match the speed of their tires with the top speed of their vehicle."

Laugh if you want, but I know the SRT6/SLK32/C32 is capable of 186+ mph speeds with current available mods, but for most people "only" 186 will be more than enough.