They generally look horrible, but that's probably due to people using the wrong kind with the wrong spread, or super bright ones where they shouldn't be super bright....or COLORED LEDs everwhere which look like ass - I'm talking to you, Pudluther! Your BSD looks terrible. Mine is incandescent and is WAY brighter than yours and has correct, natural color. Putting blue LEDs on a game with blue art is retarded...you will SEE BLUE when WHITE light is illuminating it - PLUS all the other colors on the playfield....blue light will just monochrome everything to blue. Awful, just awful. The only LEDs on my BSD are in the pops & Drac's coffin cuz its a pain to change them....and one in the insert under the coffin ramp since it gets a bit murky under there.
When I see people making their Whitewaters ALL BLUE...I just shake my head. Pinheads do not understand color theory AT ALL. There's a AFM for sale on RGP right now - there are green LEDs behind the green martians on the backglass. Those martians have all sorts of intricate detail and subtle shading and color...a white light brings that out...a green light turns them pure monochrome green!!!
Generally the only LEDs I use are in spotlights and pops. Clean games with working lights are plenty bright.