Joel Kallman of the APEX development team did a nice post about this new release here.
He not only mentions some of the new features that come with APEX 4.1, but also explains that jQuery Mobile will come shortly after this release.
I played with the Early Adopter release of APEX 4.1 for some time and liked it very much. I also tried some of my applications and they ran without that many issues. You might want to read on the changed behaviour documentation of APEX 4.1 first.
As the production release is now available, I upgraded one of my APEX 4.0 instance to APEX 4.1 without any issues today (if you need to rerun the ACL privileges, read this). It took less than 15 minutes to do that.
I don't tend to wait too long before I upgrade to the latest release of APEX as with every release you get so many more enhancements and new features, it makes it really worthwhile.
I like the new UI of the Builder very much too and look forward to use APEX 4.1 day in and day out.