If you have only one computer and it's your production machine, don't upgrade. The 10.5 upgrade is a big one--not a small update, not a few bug fixes. Lots of stuff gets broken and if you need to keep getting your work done, just wait. Let a few dot releases ease things out.
If you work with Adobe software and need your software to work reliably, don't upgrade. Apple didn't get its gold master out to third party developers in time for the upgrade path to proceed smoothly. Everything was rush, rush, rush. Developers simply did not have the time to work with the final product and make sure their apps would be compatible. If you need Acrobat (and I do) or In Design, you need Tiger. Don't upgrade to Leopard.
If you work with Windows, don't upgrade. Windows networking isn't working so good, according to several of our panelists who need to connect to Win machines on a regular basis.
If you have a lot of system customizations, don't upgrade. Many customizer tools like APE (application enhancer) are broken with the Leopard upgrade. You can do a clean Leopard install but you probably don't want to upgrade at this time.
So what do you do if you really want to give Leopard a spin? I recommend dual booting. Keep your Tiger installation now for the real work and add a Leopard partition to play.