BizTalk Server's key benefit is that it provides a lot of 'plumbing' around deployment, management, performance, and scalability. Through Visual Studio, it also provides a comprehensive framework for developing solutions, often with relatively little code.
The frustration and steep learning curve that others mention often comes from using BizTalk for the wrong purpose and from a misunderstanding about how to work with BizTalk and message-oriented systems in general. The learning curve is not as steep as most people suggest - the essential part of the underlying learning actually focuses on changing thinking from a procedural approach to a stateless message-based approach.
A drawback people often cite is cost. The sticker price can seem to be quite high; however, this is cheap in comparison to the amount you'd spend on developing and supporting features on your own.
Before you consider alternatives, or even consider BizTalk server, you should consider your organization's approach to integration and it's long term goals. BizTalk Server is great in cases where you want to integrate systems using a hub and spoke model where BizTalk orchestrates the activities of many applications.
There are other integration models too - one of the more popular ones is a distributed bus (don't confuse this with the term "Enterprise Service Bus" or ESB). You can also get BizTalk to work as a distributed bus and there are alternative solutions that provide more direct support. One of the alternate solutions is an open source solution called nServiceBus.
When considering whether to use a commercial product like BizTalk, verses something else (open source or developed in house), also consider maintenance and enhancements and the availability of the necessary skill-set in the marketplace.
I wrote some articles that go into more detail about the points I discussed here - here are the links:
