This thread is particularly relevant to me, since I've recently pulled out a sizable number of world coins that I've purchased over the last ten years (really, more like "hoarded"). For some of the better pieces, I am astonished at how much they have increased in value based on auction records. I have a very few high-quality Chinese dollars and half-dollars that are up tenfold, and for that reason, they are going to NGC for certification. I also collect Mexican 8-reales and pesos, as well as 2-reales, and in my opinion, the NGC values are too low by maybe 50% for better circulated grade coins. On the other hand, where I see the biggest errors in being too high, are for mint state coins and more recent "modern" coins.
I do believe NGC's price guide offers a tremendous resource if you look at it as "relative values", i.e. looking for big jumps between grades, or distinguishing common vs. key dates.