I was asked yesterday if I notice any difference in performance: yes and no. Yes, because all the display stuff is noticeably faster - especially browsers get their content displayed much quicker. There is a significant improvement here.
No, because the nice - yet useful - features like Spotlight and FileVault (which weren't available in Jaguar) also consume some of the system performance. Especially more intensive memory operations in my home directory are affected. On the other hand, the features are really useful, so I'm happy to pay the performance price.
So overall, the display is faster and the rest is not slower. Considering that I'm two major releases ahead on the same hardware as before (867 Mhz 12" PowerBook with 640 MB memory), this is a good result. The leap over two Windows versions certainly requires more frequent hardware upgrades to remain enjoyable.