Mule ESB is lightweight and fast. It's simple to conceive a flow using the Anypoint Studio IDE. It's also easy to deploy a SOAP or REST (RAML support on Anypoint platform is very useful) based service.
A lot of free connectors are available (Twitter, Google, GitHub, Dropbox and so on).
Provides a lot of out of the box modules to implement modern enterprise integration patterns like messaging, file transfer, database poll, web service proxy, validator, bridge, message routing and so on.
Mule ESB core engine relies heavily on Spring Framework, hence it works well with user-defined Spring components.
First class support of JMS is appreciated.
Overall community support is great. Mule ESB core is Open Source (https://github.com/mulesoft/mule).
Flows testability has improved, especially since the new MUnit module has been released. You can now test Mule ESB flows directly using XML.
Mule ESB is stable in production and is easy to monitor with JMX.
Anypoint platform is evolving and Mule ESB documentation is getting better.
Some features (DataWeave, Batch module, Management Console, Clustering) are only available with the Enterprise version.
Mule ESB lacks of documentation when it comes to create a custom connector.
No support (graphic representation of flows) when using other IDE (IntelliJ Idea).