LED's produce a very harsh light compared to bulbs.  It's kind of like comparing a high-output flourescent to an incandescent bulb.  Plus pinball machines duty-cycle the lights.  This means they turn they on and off very quickly.  So quickly in fact that regular bulbs don't have a chance to cool-down (turn off) where leds will turn completely off.  The causes leds to flicker.  Personally I think led lights look awesome in certain machines, demolition man, t2, t3, and other "futuristic" machines.  I don't think they look good in machines like white water, fish tales, etc.  Now, a few strategically placed LCD in those machines look great.  On my TZ I have leds that when not lit are frosted white globes.  When lit they are red, yellow,green, etc.  These are awesome for mode start lights imo.

Now all this is my opinion.  What I would suggest is that you buy a dozen or so light and try them out to see if you like them.  Definately do this before you spend 100 bucks to replace all the lights in a machine.

I agree 100% Chuck.  That why I like them in inserts mostly.  On the T2, all GI lighting is 100% factory.  In LAH, I don't think I would do any more.  I only repaced the globes in white inserts (white LEDs), the greens and a couple of spare white ones on the crane.  (also experimented with white in red inset).  As for the GI and rest of game, will be left as is.
I'm not sure about the kits you can buy from OS, whether they include GI lighting, or just playfield inserts and backglass.

One thing I found whilst experimenting is that if you have them behind stuff, ie mountains, plastics etc...if there are any inperfections with them (which they'll have) you will really notice...