Compress PDF with adjustable ratio
As Guy mentioned, ColorSync Utility is what your looking for. For me the standard compression was also too little resolution &amp; too lossy compression. So I created a new filter in ColorSync - which then becomes available in e.g. Preview: Resolution 200 dpi, jpeg quality ~75%  

Step 1: Open ColorSync Utility &amp; create the new filter such as below



The above settings give me files with acceptable size and decent quality (e.g. for sending by e-mail)

Step 2: Open the PDF in Preview, then choose File > Export, Click the Quartz Filter pop-up menu, then choose the relevant filter ("Reduce File Size 200dpi" in this case)





Edit 2011-12-18: Seems for Lion it's not that straight forward regarding Preview integration. See here:


  Just copy your filter from /Users/YourName/Library/Filters/ to /Library/PDF Services/ and the customized profile will be back in preview.