Alissa The works are unrelated and set in different worlds, so you can safely pick whichever you like first. I agree with Lee Dunning though, Half a King is not completely a discontinuity, but it's very different from Abercrombie's previous books, so if you had been meaning to read him forever, The Blade Itself is your pick.
Lee Dunning Personally, I would start with "The Blade Itself". It's more representative of Mr. Abercrombie's work. I very much enjoyed "Half a King", but it is definitely geared toward a younger crowd, and as such lacks some of the complexity of the author's other works.
Softbananas I'm with the others here, The Blade Itself is top notch. I just started reading The Half King and although not quite the same it has much of the Abercrombie style and I am enjoying it very much
Pete Kim please read The Blade Itself and read everything in published chronological order! it will be very rewarding. you will get a greater feel and appreciation for the entire...existence(?) of his writing universe!
Billy would advise you read the First Law trilogy first, simply because I think they are JA's best works thus far. I've not read this title yet but I will. I have read everything else he's put out since the First Law and I tell you, nothings come close to being as good.
Luke It depends on the reader. I picked Half a King because I'm not a fan of the 'dark and gritty' fantasy that he is noted for, and this book toned that aspect down. But I still wanted to find out what else makes Abercrombie the talent he is. I wasn't disappointed.
Chris (The Genre Fiend) Agreed with the first two answerers - The Blade Itself is more indicative of Abercrombie's strengths as a writer, though Half a King is (so far, at time of writing - nearly halfway through it) still pretty good.