I have had a bad experience with store bought metal ramps. It seems that the metal to concrete contact is actually quite slippery and in fact "kicked out" the ramp dropping the car about 8 in. when driving up the ramp. I tried various schemes such as rubber sandwiched between the concrete and ramp but finally sold them as I could never make it work. Now in parts stores I see plastic ramps I much prefer jack stands with additional back up support.
Oily