We were looking for rentals in DFW this past summer and we used many, many websites to search. We found several homes that we liked, printed the pages, and brought them to realtor so that we could see them. Every house we found (within the last week) was no longer available.
So in OUR experience, online searching was practically useless. At least where and when we were looking, houses would go on the market (for rent) for a matter of days before they were gone. We viewed two homes that we really liked and had just gone on the market. We decided on our top choice at lunch and our realtor called and discovered it was already gone (from when she had checked that morning). Fortunately we were able to get our second choice (which had just gone on the market THAT DAY) but we had to hurry with the application because we were told other potential tenants were already wanting to apply.
The other thing to keep in mind is we learned realtors make very little money from setting people up with leases (compared to selling homes anyway) so many are very hesitant to spend much of their time or resources house-hunting with you.
Hopefully you can gain from our experience and find something you like. Good luck and let me know if you have any questions!
[This message has been edited by bq06 (edited 11/7/2010 2:45p).]