I'll toss my hat into the ring on this one too. Cy is indeed highly perishable, needing constant refrigeration, and even at that, it will show signs of deterioration after about 10-days of correct refrigerated storage. The aroma of fresh, compressed yeast can run from musty (like old newspapers found in a damp basement) don't ask, to an ammonia smell. These are normal for compressed yeast. A good visual indicator for old or expired yeast is its color. Dark brown and a cracked appearance are good indicators that the yeast is long in the tooth. Texturally, the yeast can be dry feeling, or it may feel somewhat gummy/slightly sticky, both are normal. As yeast ages, it dies, and glutathione is released from the cells. Glutathione is a reducing agent much like L-cysteine (PZ-44) or you can even buy "dead yeast" as a natural reducing agent. Keep this in mind if you are forced into menu labeling and find yourself in need of a reducing agent. The reducing agent breaks down a portion of the gluten, making the dough more extensible as well as making the resulting crumb structure in the baked product more tender/less chewy. From a flavor and performance point, there is no difference in any of the three yeast types (compressed, ADY, IDY) when used at the correct substitution levels, and reconstituted correctly. A lot of the "old school" bakers still like to use compressed yeast because that's what "it" is, old school, and it fits well into their concept and way of doing things.
Tom Lehmann/The Dough Doctor