Apple has just released an update to Final Cut Pro (5.1.3). Apple describes it as a maintenance release that "resolves performance and other issues." Specifically: Render File Capability - apparently, render files created on a PowerPCs and Intel-based Macs weren't playing nice
Keyboard Layout Issues - "adds several commands to the default keyboard that were missing"
Issues with Cross Dissolves in Nested Sequences - "resolves cases in which cross dissolves did not work as expected in nested sequences containing still images with adjusted motion parameters"
You can read more here.
Of course, as with any upgrade, you might wait and see how others are faring with the update before you upgrade yourself. The VersionTracker message board is one good place to look for this kind of feedback.
Also, if you move between different computers using the same project files make sure all the machines will be upgraded at the same time so that your files aren't rendered incompatible, as sometimes happens.
Finally, use extreme caution when upgrading software while in the midst of working on a project. I have seen nasty stuff happen to FCP projects when someone upgraded their system from, say, 4.5 to 5.0. I've even seen it happen with a "dot something" upgrade. Such a small incremental update like this (5.1.3) is probably okay. But I'm not going to guarantee it. Safety first: If things are working just dandy for you with 5.1.2, don't chance it. Finish that project, output it, archive it, and then do the upgrade.