In regards to the reviewer who said this product is unstable - yes, it is. That is part of the point. Using something naturally unstable, i.e. a roller with a single wheel puts considerably more stress on your abs, and engages far more muscles in order to keep you balanced. This is a very good thing! In much the same way as doing free weights in the gym will give you a much better workout, and engage a great deal more tiny stabiliser muscles than opposed to using a machine, with only one possible plane of motion - so this roller will hit your entire core much harder than using a roller with more wheels to keep you balanced.
This is a very challenging exercise to do, even for those who are reasonably fit, so take things slow, as always, and build up over time. Starting on your knees is obviously the way to go, to do a full extension and return from your toes will require some super-human conditioning!
This roller isn't made to last very long, but it does the job well, and doesn't cost the earth.