I've never experienced it first hand... but from what I hear, it's not too pleasant. Apparently there are things you can do to remove some of the water soluble impurities. Check out this link:
UKCIA Soapbar warning - cannabis conamination - don't buy soapbar!

The only way to end the distribution of contaminated Cannabis and Hashish... is simply to stop buying it. People will realize that the consistency experienced with offering a quality product... will ALWAYS outweigh the minor profits associated with pumping out some adulterated garbage once or twice.

Be careful :hippy: