Non-GC shaders and non-GC lights will produce bad renders with GC enabled.

But this can be easily fixed thanks to BB and SnarlyGribbly's scripts.

If you need some GC-lights, go here:

http://www.renderosity.com/mod/forumpro/showthread.php?message_id=4055697

SSS and IDL and light falloff won't work properly without GC. You get an "effect", but it's not realistic.

Once you get used to GC, it's also much easier to build shaders or light your scene as you don't have to compensate for errors anymore.

GC for example lets you have much more light in your renders without the dreaded "highlight burn".