Just installed the OTA (over the air) update on my N900 and the difference in speed is actually user-perceptible. Animations are smoother, rotation happens much more quickly and I could swear applications are starting faster as well. Many people are reporting improved battery life too.
There are no new major features that I’ve noticed, so of PR1.2 was a feature release, PR1.3 is very much a fine-tuning release. And despite what the few whiners on the maemo forums might say – that is not a bad thing!