I just got quite a nice deal. Used carwow as a lever, and got the dealer to beat the best carwow price by over £500, so total of just over 16% off list. Was quite happy with that, was probably helped by the fact that I wasn't intending to buy! So nearly walked away a couple of times.
Strategy I used was not mentioning carwow to start. They had 100x100x100 deal, so did test drive then got them to quote offer price, then said ah ok, well it's not a great offer, then into carwow conversation. Basically said I wasn't in a hurry, happy to wait a few months.
Ended up very happy with the deal, although I did buy a new car which I didn't intend to - whoops!