Buckley's Mixture is a formulation of cough medicine produced in Canada. Although the mixture was first formulated in 1919, it has changed formula over the years, most importantly with the addition of dextromethorphan (in some formulations). The other active ingredients are still included, although whether they are actually pharmaceutical in nature or merely a type of active placebo is a question left to the reader.
The tagline for Buckley's is "It tastes awful. And it works." And they are not exaggerating on the first bit. Cough syrup is almost proverbial in its foul taste, and Buckley's takes that reputation, and Turns it up to 11. The major ingredient that contributes to the bad taste is probably Ammonium Carbonate, which as an ammonia-based substance, makes the mixture taste like sucking on clothing drenched in stale sweat. The reasoning behind the foul tasting ingredients may be as an active placebo, that people think anything that tastes so bad must be potent indeed. It could also be a streak of conscience on the part of the Buckleys: those who wish to use the dextromethorphan for recreational purposes would not do so casually. The mixture may also work on the dwarf bread principle: people will be reminded that their coughing is not really that bad, because it is still not bad enough to drink this stuff.
With all of the initially foul taste, I found that the mixture was not as offensive as it could have been. Most cough preparations, such as Robitussin and its imitators, have sugar added, and have a nauseatingly cloying texture that is even more nauseous than the taste. Buckley's, however, seems to be less nauseating once the initial shock of the ammonia taste is gone.