Depends on the LED and where its placed.
I saw a pin backbox loaded with them, as well as the general
Illumination, with white leds. However, they gave off a Bluish Tint to
them... and it looked horrible.
On another machine, they had used some multicolor leds under some
playfield inserts that looked very nice.
Some games use leds without any insert... and are too intense to
look at.
Leds under things like slingshots (general illumination) do not look
very good because the LEDs do not spread the light out enough, as
well as have the wrong color hues.
They are nice in that they wont need replacing for a very long time..
and that they do not make playfeild destroying heat like most lamps
do.