Rode my 55mm X99s (V5 shape) today for the first time, at my local dusty and slippery indoor park (concrete floor, plywood transitions and obstacles from pyramids to ditches to miniramps and what have you).
To start the session I rode my 55mmm F4 Radials for a while to get a fresh comparison. As expected, they were fast but had the potential to slide out easily. Not a huge problem, but I don't like the super icy feeling when going as fast as possible around corners and bigger transitions.
Then I swapped to the X99s.
In a nutshell, they are f*cking great. A lot less slippery than F4s, I could push on the transitions with more confidence. And they felt fast, almost as fast as the F4s. And they powerslide just fine, absolutely no problem.
They have a sound (not all quiet like e.g. Dragons), but it's different and "softer" than on F4s -- but for me this wasn't anything distracting. They sound like real wheels.
Grinds work fine, too. Just one session in, though -- I have to test them grind-wise more, but there was no substantial stickiness. A different, a tiny bit "slower" grind on metal coping, but nothing critical.
Overall, very happy with these wheels. Looks like they'll be my winter indoor top pick. Bones have really achieved something recently.