Having bought real estate, I can say that Zillow/Trulia are both a blessing and a curse. They are great in that you can see what's been listed for a while, scan a map, and generally put a pretty interface on house searches. This I imagine is their great advantage. I also found my mortgage company through Zillow's mortgage rate search.
Their big disadvantage is that their records are not updated as fast as the conventional MLS. The house I bought recently came on the market 7 days before I made an offer. It was not on Zillow even by the time we had the contract signed. The sellers, for whatever reason, put a very reasonable (possibly even low) price on the house, and I was at the end of the search, having seen enough locations in the area to know that this was a great value.
Another random personal experience: when I first saw Zillow I remember thinking "who needs real estate agents if you have all this?" Then I got an agent to buy my first house. All I have to say is "you do not know, what you do not know." While I do wish that agents simply took a set fee instead of it being a (very large) percentage of the purchase price, they provide a hugely invaluable service. I am not saying it's impossible to buy real estate if you don't have an agent; but if you can do it, you are probably a real estate agent.