Camel, Mule, ServiceMix all support Spring, but none can hope to provide the same experience as Spring Integration, as it's developed by the same people who built Spring itself.
Of all the projects mentioned, the only one I would consider even remotely simialar to Spring Integration is Camel - the others are essentially top heavy servers. Spring Integration and Camel are APIs that make servers irrelevant. Spring Integration, specifically, is a framework that provides leading integration with Spring. The development moves fast, but the Spring Integration project tends to hold itself to a higher bar for quality of implementations. If it's not clean, and built in keeping with Spring standards, it doesn't get shipped. Spring Integration provides a POJO centric model for business logic, letting you provide a clean, natural mapping for your integration flows in the XML definition. It features an almost 1:1 mapping to the
Patterns of Enterprise Application Integration book, where key concepts in the book enjoy a simialar priority in the API. For example, a channel is a top level API concept that you'll deal with in almost everything, just as the book encourages. Camel, on the other hand, does not have as natural an API. And, frankly, it has some very clumsy API elements that - I think - speaks to its sort of urban sprawl approach to API design. Spring Integration, for example, has no <try><catch> blocks in its XML!
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