It does go a little stiff but I just rubbed my hands over it (oo er, etc) to loosen it all up and then hoovered it. You get a fine dust off of it. If I was doing it again I would use a reasonably stiff brush (but not too stiff) and give it a good going over, then hoover it. Using my hands didn't really give it a good enough going over before putting it back in the car and had to hoover it again 'in situ' which produced a little more dust (easily hoovered up though). After that it's reasonably smooth. I wouldn't say it's quite as smooth as the original (it has got a coating of paint/dye on it afterall) but to be honest it's not like you're going to be rolling around on it or walking barefoot over it Certainly nothing to concern yourself about - underfoot (or undertrainer) it feels just fine, you don't think 'This feels a little odd' or anything. It probably only feels different because I'd rubbed my hands over it before and after - anyone getting into the car for the first time wouldn't notice anything untoward, and like I said I only noticed the difference when using my hands... and how often do you rub your hands over your car carpet?!
You're doing the easiest colour swap though - grey to black. Going lighter (like I did) was a bit more of a gamble.
Oh, and give the carpet a really good clean beforehand. Plan to leave it on the washing line for at least 24hrs, and more like 48. I got some Aerial clothes washing liquid on it with plenty of water and you would not believe how dirty it was (and it needs to be clean for the dying process).