Yes, it will run along side Safari. It uses the Safari UI and so you start it, you get the Safari menu. But there's a different dock icon - darker blue than Safari. You can delete it at any time by dragging to Trash.
Pros: it's generally faster at JavaScript. It uses a new JavaScript parser. It supports newer CSS standards.
Cons: it may be unstable since it's essentially the in-development version of the Safari renderer. That said, it has never crashed on me.
At the moment, I'm using Safari 4 Developer Preview. Again, very stable, faster than Safari 3.