I find the most useful thing is the ‘max speed hold’ setting, as when there are long 50mph stretches of motorway with ‘specs’ cameras etc, I can find myself inadvertantly speeding up to over 50mph without it, just by keeping pace with other traffic!
The most useless thing is having a touchscreen to adjust all sorts of settings, when good old fashioned buttons and dials sufficed very nicely. They are also easier to use without having to take your eyes off the road to look at the dammed touchscreen!