Plusses: - Supported Ansible modules are usually more reliable and more clearly defined in terms of how they'll function than community cookbooks for Chef are - Upgrades between versions go smoothly - People can just go read your YAML and figure out what it's going to do
Minuses: - It's not code, so if you want to do something more programmatic, you're going to end up with something ugly compared to what you can do in Chef/Puppet's Ruby DSL - It's still a bit immature in terms of comparison to Chef's server offering (Ansible Tower is pretty useless), and the Kitchen/Vagrant/Serverspec stuff is also not as advanced.
Ansible seems to fit well as a replacement for the Chef Zero use case of fire-and-forget provisioning - works well inside Packer or inside of Terraform via a null_resource, but I don't tend to find myself wanting to use it for long-lived machines that in the Chef world benefitted from continually running the cookbooks to help keep config in sync.