My big worry about above ground shelters is echoed by dale. If cars are being thrown around, then above ground shelters are somewhat vulnerable. I believe the stats for deaths or injuries from tornados are not from a direct hit, it's from the debri. At the very least if a car hit an above ground shelter your ears are going to be ringing for a while, and I wouldn't want to be on the side of the wall that takes the impact. And car might actually pose less of a danger than something small and traveling at a high speed.
If it's your only option, it's decidedly still better than nothing.
If it's your only option, it's decidedly still better than nothing.