We chose Paramount because they were the only agency in the area proposing a couple of things: (1) a reduced percentage for their fees if the property sold below a certain price and (2) a pre-contract lock-in agreement between the buyer and seller. This gave me some comfort that: (1) they were working hard to ensure maximum value for us and (2) once an offer was accepted, there was less chance of things falling through without good reason (we were up against a deadline so this was important - I guess normally this would be more important to a buyer).
In getting the flat onto the market the service from Paramount was thorough and prompt. More importantly, once we had an offer in, they were excellent - their negotiation of the price and the advice we received in relation to that process was very clear and was proved to be correct.
My only gripe (and the reason I'm rating them "good" as opposed to "excellent") is that they seemed very busy and didn't have people available to show the flat on a couple of occasions. I also had some concerns when they proposed letting a prospective buyer get access to view the flat alone when we weren't in either. That said, it wasn't a major problem - Paramount were just very keen to get our flat as many viewings as possible and I would still recomment them to anyone looking to sell in the area.