Nothing really wrong with them, but the magazine article that was posted on here not too long ago (from car craft) shows that the Thorley's didn't make as much low end as long tube headers, and IMO, for the price, (and for most trucks) should.
They obviously seem to be a quality product, and that in itself should be considered when buying headers, which are notorious for being annoying in many respects. But of course, if you are sacrificing low end power in a truck that needs it, you've got to decide what makes more sense to you, for your application.