i have tryed k before, it was a great laugh, different to anything else, puts your mind in all sorts of wierd places, i had a sort of outer body experience and i was laughing my tits off, it was great.
GHB is also ok, i do it from time to time it is euphoric and relaxing, you feel talkative and uninhibited - feels a bit like a combo of booze and xtc but when it wears off it leaves you without a hangover or comedown, it can be dangerous if you take too much or if you mix it with alcohol, i have seen a couple of my friends do some very silly things after overdosing, about 4 grams is a good dose for a nice buzz which is not too messy.
never tryed nubain, its addictive i think.