Leopard is definitely a worthy upgrade that's chock full of cool new features and eye candy. Much has already been written about that already.
One gotcha you need to look out for is incompatibilities with your mission critical applications. Before you run out an upgrade to Mac OS 10.5, make a short list of the applications that you rely on and use every day and check with every vendor about their Leopard compatibility.
One application that got me is Filemaker Pro Advanced 9. After installed Leopard over the weekend, I discovered that FMPA won't launch. It just throws a dialog saying "A general activation error has occurred (50040)."According to a note posted on Filemaker.com the company is targeting a Leopard-compatible downloadable update to be available on November 19, 2007.
The company claims that there are two known issues with FileMaker Pro 9 and FileMaker Pro 9 Advanced with Leopard: The Instant Web Publishing feature does not work and each language version of FMP 9 and FMPA 9 works only when the Mac OS System Preference "International Formats Region" is set to a specific region.
I think that I may have found a third issue: based on the error message above FMPA 9 appears to have an issue with the company's activation servers with Leopard.
What application goofiness have you discovered now that you've installed Leopard?