I've used the Thomson, Pace, and Samsung boxes.
Thomson > Slow as anything to use, such a massive delay when selecting planner etc, but in 12 months of ownership, it only had to be fully turned off and back on again about 3 times.
Pace > Fast EPG to use, the weird purple screen thing when using HDMI, although I have fixed this on mine by setting something on the TV, smartHDMI or something to OFF.
Picture quality very good. Fairly quiet.
Samsung > Fast EPG, I think the best SD picture of the lot, HD on par with the pace box.
Does have a little bit of a whine noise when turned off in standby occasionally!
Not tried the Amstrad box, but if it's anything like the SKY+ boxes, i'd steer clear, but im sure they must have improved since then.