Having built, managed and own Storage Sheds they are like any investment, some positive points and some on the down side.

Upside 

They share finance risk with numerous tenants , well managed high occupancy preferred.

Tenants complaints reduced due to no services to repair.

Trade customers can stay for many years.

Minimal maintence costs.  

Some states Capital Gains deferred due to classification as "a business". (check with CPA) 

Downside

Recommend onsite manager (possible wages) Security issues, Markerting

Hope this helps,overall sound investment in my view.