IAM is free of cost. It is Payment Card Industry Data Security Standard (PCI-DSS) compliant, so you can run your credit card application and store credit card information using IAM. It is also eventually consistent, meaning any change you make in IAM would be propagated across multiple AWS data centers: this propagation could take a few milliseconds. So design your application and architecture keeping this feature in mind. IAM integrates with various AWS services so you can define fine grain access control for these services.
Let us look at other features of IAM that make it such a widely used, powerful, and versatile AWS service. As a matter of fact, if you have an AWS account and you want to use resources in your AWS account, you have to pass through IAM in one way or other, there's no two ways about it!