We have previously used Microsoft BizTalk Server. I've also worked with webMethods and a little bit of MuleSoft.

This is the solution we currently implement for our customers. 

We are also using IBM API Connect.

The differences are the portal and standards.

DataPower is a bit easier to work with. Also the analytics. 

It has a set of tools that make life easier when it comes to developing APIs.

I've used Message Broker from IBM, Oracle Service Bus, and TIBCO.

DataPower has the advantage in that it's more limited in capabilities than the other products, so it's harder to accidentally write applications as part of your integration. If you follow strict patterns then your business logic will stay in the applications and your integration logic will only be on the DataPower. Like all devices, you can actually go to extra effort to put application logic into the DataPower, but that just ruins the device for its intended purpose.

Previously, we were using the webMethods server.

We did not previously use a different solution. 

We did not use another solution prior to this one.

I used Apigee earlier.

I've used other API solutions in the past. IBM is a huge organization in comparison to most of the competition. It has massive development and offers very high-quality products. Most others don't really compare.