I watched Appaloosa last night on Blu-Ray and it was of spectacular quality, like just about all of the Blu-Ray titles I've seen.
Every now and then you'll see one that's sub-par, but Blu-Ray compared to regular DVD is like night and day.
One additional thing that comes into play with Blu-Ray is the new audio formats. Both True-HD & DTS Master Audio (often seen as DTS-MA) provide lossless soundtracks that are bit-for-bit duplicates of the original master soundtracks. The older Dolby Digital and DTS soundtracks are very good, but they use compression algorithms that squeeze out parts of the soundtrack in order to conserve space on the DVD. Blu-Ray has sufficient space for both the HD movie, and the original soundtrack.