HICAS shouldnt be kicking in before 80kph, tho it'll certainly give u error messages before that speed
Mines currently stuffed, and I'm in the process of getting the tools to fix it properly (first and foremost being a conzult interface)
When it wasnt broken it made high speed driving very very responsive... most people aren't really ready for that swift turn in, they expect the front wheels will take a fraction of a second to actually bite... basically it shifts the arse end of the car around quicker than most people expect...
When its stuffed (like mine is now) it can be the bane of yr existence. It'll mess with the power steering if the hicas computer malfunctions, you'll constantly have an annoying light on after 15 minutes, and then you'll get random actions happening if you do actually take it past 80kph within the first 15 minutes... like cruising down a highway in a straight line and ur rear wheels turn that 1'.... it makes a huge difference, especially to the amount of nappy san you have to add to your soiled undergarments...
However, nissan put the hicas there for a reason. Its true enuff a pro driver could go faster without it, but generally such cars have a stupendous amount of mods that'll make the car powerfull enuff that it wont be needed....
And, I'm far far far from a race driver. So yeah, happy to fix the hicas. Just need to pay my laptop off then the cable is comin' And then I actually get to >fix< the root cause (and I have a spare hicas computer just in case woot)
-D