Mac OSX Leopard brings  a ton of new features to MacOSX but it still needs to work out some kinks.  The major drawback is that you have to do a complete CLEAN INSTALL in order to avoid major conflicts between the new OS & the applications.  Often times If you are not VERY careful, then you may lose some or all of your programs.  Personally, I would wait to get OS X 10.5 until some of the kinks & problems have been worked out.  When OSX was first introduced, we waited between 4 & 6 months to get the new OS, which allowed Mac to work out the kinks and refine the Operating System.