It has recently started becoming popular quite organically, so I might just write a blog post on it soon.
I've found depending on how strict your change management policies are, IAM creds can collect cruft over time as people push new policies in ad-hoc. So iamy is handy for such a situation
- iamy can sync in both directions - pull and push IAM config. So you can easily pull down the ad-hoc changes
- In order to use CFN you need to have access, so there is a chicken-egg scenario if you want to manage ALL users in config
- iamy gives you a nice execution plan of aws cli commands, CFN can be opaque
And iamy does ignore any resource managed by CFN, so it works well as complimentary tool.
We consolidated users into a bastion account, ran into annoyances with CFN, and have been using iamy ever since for change management across all our accounts (more of a writeup at https://99designs.com.au/tech-blog/blog/2015/10/26/aws-vault...)