Since the HD 5750 sits within £20 of the more powerful 5770, one would hope that it's aftermarket cooler doesn't bridge that gap even more. As it stands, I feel that the 5770 is worth that extra money for it's additional stream processors and higher clock speeds.
I too would also have preferred a heatsink with a shroud that ducts air outside of the case, like the reference cooling solution. That's not to say that it'll cool well but I'd be interested to see what happens to ambient temperatures and noise levels when compared to the reference card.