Picture quality is good, but not great; SD channels are upscaled slightly, but there's plenty of picture noise and some blocking. We also spotted a lot of jagged lines all over images, suggesting that the GDB300HD is not the '1080p upscaler' it's mooted to be.
There's no such problem with high definition channels though, with ITV 1 HD, BBC HD and 4HD swimming in an impressive amount of detail. As well as being studded with vibrant and well saturated blocks of colour, we noticed that pictures were a lot smoother with no jagged edges or picture noise, though close-ups don't seem to enjoy as much detail as they should.
However, in this regard it compares reasonably well to most other Freeview HD boxes on sale.