I think the FUSE set of products is outstanding. I've personally been using ServiceMix on-and-off for the last 4 years or so, with great success. That said, I am always just a little leery of the business model used by IONA, which like Mule, may not always make the most advanced enterprise features FOSS, but instead try and sell you on a commercial alternative or add-ons to provide required functionality. JBoss and WSO2's model, on the other hand, I find a bit more reassuring. For the most part, they don't make any money by selling commercial alternatives to their open source sponsored projects, but instead make revenue on support and consulting.
In the book, I cover Apache Synapse as the ESB, middleware and web mediation solution. In part, it's due to the considerations I mention above, but also that I prefer the lightweight and easy-to-use nature of Synapse. I'm a huge proponent of ESB technology, but I don't think it's the swiss-knife for everything SOA. I think services should be built using a framework such as SCA (Apache Tuscany or Fabric3).