BW5 has the advantage of fast development, and more importantly to me, I can train a developer already familiar with basic computer science concepts to us BW5 in almost no time. Our newest developer I trained in two-hour sessions, and after maybe 4 or 5 of these session, he was able to handle projects mostly on his own.
One especially brilliant component of BW is the mapping. It's very easy to translate data and implement complex logic using xpath.
BW6 also has powerful mapping. It's one advantage is easy implementation through TEA.
The main issue with BW5 is that the designer is a little clunky, although reliable and relatively light. You can't ctrl+z. Your option when making a mistake is pulling from the repository. BW5 processes are also pretty complicated to implement, start, and stop on the server.
BW6 is not ready for release, although it's been out for over 4 years. I've been working with it for over a year now, and have installed numerous versions, each addressing a list of bugs. My team engineers integration and business automation for our global multi-billion dollar company. We are currently converting the old point-to-point systems to a Tibco driven SOA architecture. Our intention last year was to convert our suite of applications (~200) to BW6, and implement all new apps in BW6. However, after experiencing all the issues with BW6, we've changed course. Today, for us, BW6 is a REST interface, and everything else is done in BW5.
The main issue with BW6 is slow development due to the Eclipse based designer that needs further development. As noted, the only type of interface I currently implement with BW6 is REST. Even this is dangerous though because issues with BW6 can still hold up a whole project while I try to solve Tibco bugs. Today the designer is randomly throwing up java.lang.NullPointer.Exception boxes, in addition to my project suddenly not working, giving me only an error message: PVM-MODEL-0 (No message; locale=default). I've tried to submit a support ticket with Tibco, but that doesn't seem to be working on their website. I'm about to give them a phone call. They're support people are good, but at this point, we're about equally skilled in solving these errors.
BW5 is good, but if you decide to try BW6, understand that you will need a lot of smart people around and development will be slow.
We use BW5 to automate data exchange between systems. For example, data sharing between SaaS, ERP, and local or network systems, some custom some licensed.