HD boxes provide a good clear picture - when the source of that picture is good...
many programmes are "upscaled" from standard definition to HD.
and there are two formats for HD, 720 and 1080 - refering to the number of "lines of data" on the screen. so these formats may vary from one HD channel to another HD channel.
The first few Sky HD boxes did have a few teething problems, like overheating and hard drive crashes, but i think many of the problems ahave been resolved in the latest batch. Pace have started to supply Sky HD boxes, but it is too ealy to say how they are performing and if they have similar problems.