I've been using it as a way to "ease" into my sets and I actually think it is a very valuable training tool. of course, for those who compete, as you get close to tournaments you might want to go back to "normal" lengths, but for those who don't or for those who are just really trying to get to the next line, IMO it is what makes more sense.
I must confess I don't get the "this is not a real line, you'd better slow the boat down" argument, as, IMO, one could answer "that's not a real speed, you might better shorten just a bit" ;) ? (No offense intended, sincerely)
Last year I used to tweak with the boat's speed between 32-33-34 mph, and was less consistant than this year, I now only ski 34mph, start longer (22 or even 15 off if i'm really tired), but consistently go 15 - 18.5 - 22 - 25 - 28 - 30.5off, and good days 32off.
The true difference with tweaking with the speed is, as others have said it, you keep the same rythm/timing.
Now for rope reviews, I've been using the masterline progressor, which is good but the mid loops seem to "undo" after a while as they are just pieces of rope "inserted" inside the initial rope, they come out after a while, I'll post a pic as soon as I can take one.