Netflix ICE has a few quirks but overall it's a pretty useful free tool.
Unfortunately, the project has been largely abandoned and the current version in the Netflix GitHub repository won't work out of the box for many companies due to lack of support for new AWS regions, instances, reservation types, and services, as well as showstopping bugs (such as https://github.com/Netflix/ice/issues/210). In order to effectively use ICE today most users will need to maintain their own patched fork.
It's been very helpful to notice and track down extra costs as they come in