Many things. The feel is different (typing is more pleasant - hard to describe but it's clear when you compare them next to each other's), it has one more keyboard row, and I also like the fact that there is a color scheme: some keys pop out visually so it's easier to differentiate the keys compared to most keyboards nowadays where everything is black and boring.
Nope, the 520, 420, 220 had a different design
Curious, what's great about it?
Many things. The feel is different (typing is more pleasant - hard to describe but it's clear when you compare them next to each other's), it has one more keyboard row, and I also like the fact that there is a color scheme: some keys pop out visually so it's easier to differentiate the keys compared to most keyboards nowadays where everything is black and boring.