I've been running NetBeans for several months on 5.0. Today I built the development version of 4.1 from scratch and then ran it. The IDE was slower and ugly, and I thought "oh no, what have they broken?"

Then I realized what I did ("Doh!"): the build requires that you set your JAVA_HOME to 1.4, and I forgot to reset back to 5.0 before running it. What a difference the new JDK makes! NetBeans is faster, prettier, easier to use. Sometimes you don't realize how good the 5.0 JDK is until you forget to use it.