Message brokering across different systems, with transactionality and the ability to have fine tuned control over what happens using Java (or other languages), instead of a heavy, proprietary languages. One situation that it doesn't fit very well (as far as I have experienced) is when your workflow requires significant data mapping. While possible when using Java tooling, some other visual data mapping tools in other integration frameworks are easier to work with.
BizTalk is well suited as middleware. Where you wish to translate an input file into an output file and send it to some endpoint. In our case, we used it to convert and send files to SAP. In many ways, it very flexible, and you can do almost anything you want with it. In many ways, it's a better solution than your SAP XI or PI as middleware, since it's much less expensive, and allows you do interface with non-SAP systems.
Camel has an easy learning curve. It is fairly well documented and there are about 5-6 books on Camel.
There is a large user group and blogs devoted to all things Camel and the developers of Camel provide quick answers and have also been very quick to patch Camel, when bugs are reported.
Camel integrates well with well known frameworks like Spring, and other middleware products like Apache Karaf and Servicemix.
There are over 150 components for the Camel framework that help integrate with diverse software platforms.
It is very user friendly. Users can change rules during run time and change workflow.
Huge capacity for queueing messages. It supports all types of adapters like Oracle, Salesforce, SMTP, FTP, etc. Also users can built custom adaptors.
If users want to dynamically deploy their solution without any downtime, this is a perfect solution. BizTalk will be a good fit, especially for public-facing websites.
Well-proven in the market. I used it when developing a website for Virgin Trains, catering more than 800K user requests per day.
BizTalk Server has been supported for more than 15 years. It is well proven in the market. Microsoft has provided excellent support with technical issues.
If you are looking for a Java-based open source low cost equivalent to webMethods or Azure Logic Apps, Apache Camel is an excellent choice as it is mature and widely deployed, and included in many vendored Java application servers too such as Redhat JBoss EAP. Apache Camel is lacking on the GUI tooling side compared to commercial products such as webMethods or Azure Logic Apps.
BizTalk was selected here mainly because it is easy to integrate to a .NET application (most of them are Web Service, WCF SOAP, WCF REST and Web API) and many backend databases are Microsoft SQL Server. Another benefit is that the monitoring job is easy to set up and centralize with other .NET application monitoring jobs.
Very fast time to market in that so many components are available to use immediately.
Error handling mechanisms and patterns of practice are robust and easy to use which in turn has made our application more robust from the start, so fewer bugs.
However, testing and debugging routes is more challenging than working is standard Java so that takes more time (less time than writing the components from scratch).
Most people don't know Camel coming in and many junior developers find it overwhelming and are not enthusiastic to learn it. So finding people that want to develop/maintain it is a challenge.
A positive impact has been the quicker turnaround time of a part request and that part showing up in SAP using Biztalk as middleware.
A somewhat negative impact has been the somewhat insufficient error logging/message capture settings that Biztalk provide. This has caused occasional delays when attempted to create parts for the business.
A somewhat negative impact has been the need to have a specialized developer who understands Biztalk to troubleshoot issues with the Biztalk and SAP interaction when creating parts, and when adding new fields to the parts.
