I haven't tried Zillow, but my wife and I have done Realtor.com. My suggestion about Realtor.com - don't touch them with a 10 foot pole. They put us in a 12 month contract for $400/month (I hear most people pay a lot more) for a zip code that is supposed to produce an "average of $757,000 leads" due to the average property price there. We've been in it now for 4 months, paid $1,600, received 27 of our contractual 48 leads and the highest lead we've got was $120,000. Most people already had agents, some were just browsing, others owned the property they were inquiring about and wanted to test the site. All of these leads came out of our 48-50 leads we are promised a year.
We tried to cancel our contract due to it not being anything as advertised and $1,600 is a lot to us. We have two toddlers and another due in December. We thought it would be doable if it gave us business, but so far we have flushed that money away. We called to cancel and they said short of death, we can't get out of the contract. The "customer service rep" laughed at my wife when she complained about our service we were getting.
I understand that lead quality can't be guaranteed, but don't sell us something, put an average number on it, we get no where near that number, and not let us back out. We filed a complaint with the BBB. We've requested our contract for us to send to our lawyer 4 times. They've yet to send it. Realtor.com feels like a business ran by the mob.
Anyways, sorry for the rant... just be wary. If Zillow is anything like that, God help you. I've heard better things about Zillow though.