99-0 chillin mix was a popular blend sold by PAH. Originally, it was one of the few alternatives to spice. It differs in a couple ways. It would be fair to say that spice is more like an indica cannabis high, and 99-0 is more of a sativa high. It is reported to contain a cannabinoid agonist like all spice alternatives. It probably contains more JWH-018 than Spice which gives a very strong sativa high.
99-0 chillin was less potent than cannabis on a bowl for bowl basis, but on a gram for gram basis it may be on par. It is a very smooth smoking blend that is not harsh. It is commonly described as having more of a mind altering effect than spice, but far less "body buzz" effects. Sometimes spice has been described as leaving a body load the next day. Spice very well may be slightly more potent but some have said that they do not enjoy any after-effects after smoking. Either way this, along with the well known spice do live up to most of their claims and are both most likely the best known true alternatives on the market.
Chillin 99-0 was discontinued and replaced with Chillin XXX, a more potent alternative. Later Chillin XXX was replaced with ZoHai SX, much to many consumers discontent. ZoHai SX was a potent Spice alternative however it is unlike the famed Chillin XXX in potency as well as effects to an extent.