Apple's Logic Studio has been an essential tool for many musicians for years now. For a long time, Logic has been arguably the leading software package for MIDI composition and sequencing and in Logic 9, the audio processing features have received a welcome boost.
As a user of Logic 8, the killer feature in Logic 9 for me was the 'Flex Time' audio feature. It allows audio content to be stretched and squeezed in a non destructive way. Perfect for fixing a few misplaced snare hits in a drum recording! You can also quantize audio in the same way that you could previously with MIDI content.
If I'm honest, there is a lot that is completely unchanged from Logic 8. But Logic 8 was great, so there were only a handful of areas that needed much attention.
Logic 9 might seem like a big investment but there is a huge amount of content included. And for me the full fat 'Logic Studio' edition was worth the extra investment over 'Logic Express'. I recommend that you study the differences between the two carefully before you make your decision!