ASRock IS well known, just not for graphics cards. They are up there with ASUS when it comes to motherboards. Since ASRock has only been making graphics cards since about the past few months, it's probably hard to say. Searching for and reading reviews of the specific model should tell you much about build quality and performance though.
Personally, I like that ASRock is making cards now, but I think it will take a while before they get the process down to where they can actually offer products in line with what these others, who've mostly been doing it for a very long time, can offer.
Other than that, I wouldn't have much reservation about one of their cards as everything else they do is done well.