My experience with BizTalk and doing B2B integrations is that most organizations do not truly do schema first design or fully understand xml standards for that matter. Most tend to weave objects and hope they materialize into meaninful schemas. In an enterprise environment, this is backwards.
BizTalk does have a learning curve, but once you get it you are rewarded with durability, performance, true scalability, and extensibility. Like most have said though, it best to make sure it meets your needs and contort your needs to BizTalk.
In the past I have worked with BizTalk 2004 through 2009, and another product called webMethods.
