Camel is one of the best lightweight integration frameworks fo Java. It requires some time to learn how to develop routes, handle errors and so on, but with some experience you can write almost any integration pattern in just few lines of code. Other disadvantages include: some api breaking changes between versions and lack of persistence layer in few pattern implementations. You can use Camel in any Java app server, but SMX4 seems to be most integrated. Generally - great framework!
I have mixed feelings with Servicemix. Introducing JBI Messaging API is good move, but deployment model with all that SA/SU is painful for developer. Servicemix 4.x introduced OSGi and Spring deployment but it's still rather unstable despite quite mature 4.4.x release. The Fuse team is doing great job (with recent Fabric release) , but Apache version's decumentation and release schedule needs to be improved. And is Servicemix something more than Camel/CXF container? Maybe with some monitoring/governance tools (but they're still missing...)