Webkit is simply the engine behind Safari.... Safari is the GUI, Webkit is the HTML rendering engine. Webkit is opensource and is also being used in products by Nokia and some new Adobe thing. By downloading the latest WebKit you can try out the newest changes and get a preview of what HTML rendering features Mac OS X 10.5 Leopard will have.
Keep in mind that Webkit is just the engine. The Safari GUI does not change when you try the WebKit download. The new features will be in the rendering of the HTML only (more CSS3 features, faster, etc) but no new application/gui features.
As for stability, the only way to know is to try it out. For some of the AJAX/Javascript heavy sites I go to, I often find these newest nightly builds to be way more stable than older, released, "stable" builds. YMMV.