The new version of Enterprise Library has some interesting implied additions and constraints. I have used the frameworks before and like them for the most part, they keep the structure of a large application from looking like slop. I took a good look at it and I am impressed how much better the library gets in each release. In our organization however, we have chosen not to use the framework because we used the first few and converting from 1.0 or 1.1 to 2.0 to 3.1 was a royal pain. Several applications used 3 and 4 but they soon converted to the Castle framework because the Castle team seemed to be making more progress. Both have pros and cons which you may assess for yourself if your interest level is high enough.
What I do want to note is the lack of expressed Windows XP support with the Enterprise Library 5. Here is the beginning of the system requirements section:
I saw a comment asking if Windows XP was supported and the reply was that if it works, it works.
There are a ton of breaking changes this time, but none appear to be outside the realm of making things better in the long run. There is also a Migration Guide as usual to help in the transition process.
Great job on the release and I hope to see more good things from the Enterprise Library in the future.