exciter is very nice to produce a certain kind of harmonic fatness to the bass, some kind of ringyness and definedness to the mids, sharpness to the highs, and generally make transients bang in stronger. also you can drastically alter the stereo image by using the mid/side mode, especially on the highs. Let some loop running and tune for a while, you will see, expecially with headphones!
yes I often have it going subtle on the whole mix. you know, it brings stuff onto the foreground. It exaggerates certain features, and that makes the listener able to enjoy them easier. I think electronic music is rare, that doesn't have such or similar devices going on in mix/master, music not having such stuff going on can sound very dull.
I planned experimenting with using in the mix only on certain tracks, but selectively in a way so I can tune the effect for each instrument. Instruments that don't have exciters going, will naturally sound weaker and like from the background. I think it might help to do more complex mixes with stuff going on in background layers.
It is interesting to think where in chain the exciter sits. It can generate quite peaky action from transients, that must be dealt with. Also like with distortions, any further effects will also shape the harmonics, while when the exciter is at the end of a chain, it will excite also actions that generate features earlier in chain.
I think stacking is difficult with exciters unless you know what to expect for some experimental purpose, like with stacking distortions the degradation will build up and make sound very scratchy or muddy in the bass.
I also think it is a bit unfortunate move in the effect with high intensity/sharpness going on...for example if you excite the bass (or mids) very strong, it will generate lots of tearing and square wave like harmonics to the sound, dirty stuff. I think it will sound much better in those settings, if you for example low pass filter the harmonics so only the lower ones, maybe to just some margin above the bass (or mid) freqs you are exciting, are passed, but not the gritty higher freq stuff.