I used Acorn to find my house, and they were very good, but peoples' opinions of estate agents probably aren't going to be that useful to you as it will depend on the person they spoke to (who may or may not be representative of all the people at the agency).
My advice, for what it's worth, is to search findaproperty and primelocation daily and, on primelocation, setup email alerts with your search criteria. I would also spend time going door-to-door and registering in person with all the agents. The market is (apparently) slowing so agents should be all over you and want to get you viewing properties.
Out of interest, which area are you interested in? Some posters may have local tips? There are some fine new devs near me in Bermondsey.
It can be hard to find places to rent round here. I found that no estate agent had more than one suitable place on the books (1-bed, not scummy, SE1), and by the time you show up for something you found on the internet it's gone.
I (and other people I know) just did a day or two of the agents on BHS and picked one of the bunch they saw and were available that day.
If you're more patient and have time to kill, there are other options...