We used Winkworth a couple of years ago to buy a house. We weren't particularly impressed. Despite registering with them, they failed to inform us of a property that exactly matched our specifications. We only saw it because we happened to walk past the window.
All through the subsequent sale process, Winkworth were never on top of what was going on and never had any idea when processes were likely to complete, what needed to happen next and so on. We were lucky to be directly in touch with the seller, so we could work out where the hold ups were (often it was the solicitors) and take action. We felt Winkworth didn't earn it's commission. It was hard to see what value they provided apart from the window that we saw the property in. As buyers, of course, it wasn't our problem but it certainly put us off using them when we come to sell our property.
It's hard to comment on value for money etc. as we don't know how much the seller paid.