You can do a LOT with speedlights. I own 5 speedlights (600, 580ii, YN560ii) plus 3 Einsteins and I use the speedlights all of the time. The only time I use eTTL is with my on-camera flash. My remote speedlights are always manually triggered only, so that means I can ignore all of the frills of the 600 and just use flashes with similar output and sufficient power to achieve the look I want. That said, I could easily replace all of my speedlights (the remote ones that is) with YN560-II's at $74/ea and would barely notice a difference from $500+ Canon versions (same power output). The YN's do have an annoying powersaver feature, but it's manageable. Get a set of inexpensive triggers (I recommend the Phottix Stratos II, but you're already invested in YN models), and you now have a multi-light off-camera setup with tons of possibilities.
For your on-camera flash, keep a Canon flash because you'll want the eTTL and multitude of options. For remotes however, there just isn't much need to spend a lot.