I got a chance to start looking through Knights of the Grail over the weekend, and on the whole my impressions have been good. While Brettonia does have a very different feel from the Empire, it still seems like the same 'world', albeit with a more conservative and repressive aristocracy. The sections on Brettonian society and the profiles of the individual duchies are the highlights of the book. The new careers and monsters are likely the weakest part. More detailed maps would have been nice, as well as a few more location names, but not a major gripe there. The section on Grail Knights is reviled by some WFRP old-timers as an unwelcome intrusion of high-fantasy power from the wargame, but it doesn't dominate the book, and there's plenty of leeway for GMs to decide how much importance it is going to have on their game.
One of the biggest changes from the v1 background material is that Brettonia feels like a much less urban society than the Empire. The result is that in some cases some of the major Brettonian cities have lost a bit of their flavor compared to v1 (the descriptions are much briefer), but on the other hand it makes for a very different and new gaming experience; the Empire has long been dominated by Nuln, Altdorf, and (especially) Middenheim. Having a game spend a little more time in the countryside, without throwing it straight into the wild wilderness, represents a new tack that may be interesting to explore.
Full review to follow, eventually.