+1 my brother worked for ground zero for a year or so, he built hundreds and hundreds of these things. Made right here in oklahoma.www.groundzeroshelters.com
My neighbors had the in-garage one put in recently. Looks well made and their's can hold up to 6 people I believe.
BTW- The company also has a service where they have your GPS coordinates and will come look for you after a tornado if they can't get ahold of you.