No problems encountered so far when upgrading from WordPress 1.5 to 2.0. I love the new admin theme! It makes better use of the screen in “Write Post”! The WYSIWYG editor is quite nice when creating simple posts. It seems like all installed plugins works too!
Goodies like explode, leftback, rightback, switchcase, implode, middle, trim, unique etc. can be found in this utility class.
The Commons Lang project at Apache Jakarta also has some really great stuff.
Chad Schelfhout gave me a correction for the implode(Object[], String):String method and the new method word(String, String, int):String.