Used commercial ESB and BPMs for a couple of years, recently had a chance to evaluate some open source ESBs.
WSO2: not easy to use, had difficulty even making the sample projects to work. No DataMapper tool, which is a big no-no to my projects.
Mulesoft ESB: Nice documentation, instructions easy to follow, sample projects can be built and run in a couple of minutes, nice DataMapper tool in the 3.4 version. Have not had a chance to build a relatively complex application using this. Not sure whether the community edition is good enough to be used in the Production.