I bought it on sale a while ago. My conclusion is this. You shouldn't need to use it on well recorded instruments, unless you're going for some specific kind of effect. And if you are, you have to be really careful with it because your ears quickly get used to the sound of it and when you A/B it, the original sounds awfully dull in comparison. So you think it's doing something good. But then you listen to it again in the morning and it immediately sounds too harsh and bright.
Having said that, I think it works wonders on material that is too dull sounding and which can't be made brighter with EQ (because the frequencies just aren't there). For example, an old drum loop that's a little lo-fi sounding. Aphex can really spruce something like that up. Or any recording that's a little old or muddy. Recently I wanted to use a short sample of an orchestra playing Percy Grainger, but it was a little dull and muffled sounding. I sent a little to the Aphex Exciter and it did a great job of brightening it.