I had one of these and the reason it's selling so cheap is that it doesn't work out the muscles it's designed to, and for the muscles it does stress it does so in the worst possible way. Since you're pushing in a crouched position, you're actually using your lower back muscles instead of your abs. Then, because of the way you're positioned when using the roller, you're pushing with your muscles PLUS the added weight of your body, meaning you're can strain/overextend your lower back very easily.
Here's the cheapest, easiest (heheheh), safest way to get abs: Get your body down to less than 10% body fat. Next, 3 days a week, lay on your back, raise both legs in the air as far as you can, then SLOWLY lower then down (put a pillow under your butt so you don't hurt your tailbone). Do 10 sets of these three times (or until you're strained). This is a good excersise to do while in bed due to the mattress. Works like a charm and you're not out $$$$ for gadgets that don't work. You have to be below 10% body fat or you won't be able to see your abs muscles, so lay off of the cheesburgers.