IBM - Is it really that different?

In my datacenter I have numerous customers who are "IBM" shops. They seem to use only IBM server/storage hardware and nothing else. Their networking gear is the traditional mix of the industry leaders, but their compute hardware (be it pizza boxes or blade chassis) is all IBM.

My question to the group is "whats the difference"? I have only administered an IBM server one time and that was nothing more than a Windows Small Business Server at a SMB customer's branch office. When people speak of being an IBM shop are they referring to more than just hardware? I know there is a strong relationship between my IBM focused customers and SAP development services.

Also, I understand that IBM's history of writing operating systems goes back before the dawn of man. ....maybe i just stumbled upon the answer? They run something other than Linux/Unix/Microsoft and they do it on IBM hardware? What is the name of the modern IBM OS? Does it vary by platform? If i wanted to know more than can be asked here, where might you send me?