I'm toying with the idea of one 600.
But thinking for multiple lights...getting them to work with cheaper ones.
I saw a GREAT set of classes on Creative Live, about flash, and doing it manual and how that with some rules of thumb, it wasn't that much harder to figure things out, especially for environments you have control over.
With a few charts/cards to look at...you could do a LOT manual with multiple flashes/strobes on the cheap for great results.
I'm looking to go that way to start out maybe.
All manual flash is definitely the way to start!
I've been using a 430EX II (bought used for $170), three YN560s ($60 each), and Phottix Stratos triggers ($90 for the transmitter/receiver kit). A four-light kit with on-camera ETTL capability if needed and manual radio triggering all the for the price of just one 600EX-RT. Definitely a lot of bang for the buck and this setup can take amazing photographs.
Don't hesitate to pick up a couple YN560s and get started with manual flash. Make sure you go through strobist.com.
However, I've been doing a lot more paid shoots lately and troubleshooting issues with radios, batteries, and optical slaves in front of clients (who are often in a hurry) is just not going to work for me. That's why I've decided to go with the Canon RT system.