Zillow, Redfin, and other real estate sites

Hi,

I'm new to BP and wanted to reach out and see what the community thought about the rise of Zillow, Redfin, etc.? I've read a lot of really interesting articles about them and being Seattle-based myself they are local rising companies. Should people opt to use these services over a traditional real estate agent or maybe in conjunction with an agent? Thoughts? Thanks!