This suited my purpose of an inexpensive option to start gardening early.  However, it barely made it 6 weeks before a heavy rain ripped through the plastic.  (It was not a windy day - just heavy rain).  We repaired it with tape and it's more or less been adequate to protect the tiniest seedlings an extra bit.  The shelves were sturdy enough that I will probably repurpose them when I purchase a proper greenhouse next year.