I've had 2 sets of Alloygators. The first set were grey/silver and worked very well. I admit they looked slightly odd looking at the car from the rear but were almost unnoticeable from the side.
Because they worked so well they got a bit frayed round the edges so I replaced them with a new set of black alloygators. These also looked ok but were a failure on the front wheels, as reported by Philius Maximus, they wouldn't stay in for more than a couple of hundred miles.
The black ones are a different design from the old grey ones, they seem to a slightly different shape in section and have silly little metal grips that have to superglued in. bit of a faff, really.
The upshot is that I dug the old grey ones out from the back of the garage, cleaned them up with a knife, a block plane and some sandpaper and put them back on the wheels where they are now, several thousand miles later.
A complaint on the Alloygator web site brought no response so I'm considering Trading Standards as the black ones are not fit for purpose.