IMHO, Biztalk Server is the only team in Microsoft that understands integration issues. They have delivered a product that in my opinion provides complete solution for the B2B as well as EAI scenarios. The two things I love about the product are - Orchestration and Business Rules Engine. BTS architecture is really elegant and it is built for customization from the ground up. It is possible mainly because, various components like (Pipeline components, Adapters, Ports, Message Bus etc) are loosely coupled. I could have done certain things differently - like keeping orchestratin little bit loosely coupled from the messaging etc. But, I was the first one to bitch about loose integration between orchestration and messaging in the previous version.
They really worked their tail off to deliver a quality product; their perseverance and hard-work deserves a round of applause.
While we are it, please bookmark Scott Woodgate's blog. I am sure he is interested in your feedback.