I love my Ab Lounger! I've had it for about 18 months but only started using it in earnest at the beginning of February. By "in earnest", I mean using it 3x a week. I do 3 sets of 20, but am thinking about bumping that up a bit.
So, it's only been 6 weeks or so, probably not enough to see any noticeable results, but I can definitely feel it working. I can see a little definition, a little ripple especially if I hold my stomach in, but the real abs are still hidden under other gunk.
I've done crunches on the mat, on the bench, on the ball and on a weight machine and I can honestly say, using the Ab Lounger is the only time I really "felt" my abs contracting and releasing. I'm not using my arms, my neck, my shoulders... just abs. It doesn't hurt my back or butt, either, both of which were always very uncomfortable when doing abwork on the floor or bench.
As soon as some of the gunk that's currently covering my abs has melted away, I think I'll be pleasantly surprised! At least, I hope so!
Have fun with yours!