My experience with it has been that it's a total mess, and causes problems that are difficult to diagnose and fix for non-black-belt Windows developers.
What are the use cases that make COM shine?
COM provided both local (object to object) and remote (machine to machine, process to process) communications largely transparently. With a little care your code could happily work with objects hosted anywhere in the same memory space to across the internet.
While this abstraction had its problems (the dynamics of different cases were very different) and the use of a binary format challenged interoperability it did work. And work very well (consider all the line of business applications that Visual Basic enabled).