Analyzing the value of a self storage facility

[John Ringgold]

I may have run across an owner of a self storage facility that may want to sell.  As a residential Realtor, analyzing a value on a residential property is not hard.  This is a different animal for sure and I don't get involved in the commercial space on a regular basis, however I would like some advice of putting a value on a storage unit facility.  

Would you use the income approach?

How would you factor in vacancy?

What else might I need to ask that I am not asking?