We upgraded to Java 6 almost 9 months back at my workplace. We faced no major problems working with projects which were using Spring. But, as soon as I started working on an EJB 3 project, I realized most application servers aren't even ready for Java 6 yet.
I do have multiple copies of Java installed, but testing with all these versions 1.4, 5 and 6 is a nightmare.
Now, its too late for us to switch back to Java 5. And, with Java 7 coming soon, it's surprising that some application server vendors don't yet support Java 6.
What version of Java are you using at work?
Are you facing any problems at all?