I have the race ramps with extenders which do allow you to drive extremely lowered cars up onto the ramps. They have worked well for me and get the car up higher than the rhino ramps. I hate using jack stands (a lot of trouble and I just don't trust them) and try to avoid it unless the wheels have to be off of the car.
Those wheel cribs would have been useful for me just a few weeks ago when I changed my trans fluid and needed the car up off of the ground and level. I ended up using a second set of ramps instead.
If your car is low and you can't get your jack under the car just just buy a piece of 2 x 6 board. Drive up on that and then use the jack.