I am a big advocate of compost. I make it, use it and occasionally buy it. Plants love it. Compost and worm castings are my primary sources of supplemental fertilizer -- most people are surprised to learn that I rarely use synthetic fertilizer on my 3-acre property, and none in the last two years. Unfortunately, all the commercially available compost pales in comparison to the homemade stuff -- aptly called brown gold. The good stuff is expensive, assuming you could find it for purchase. One should expect to pay $1-2+ per pound. Again, if you could find it. The 40-lb bags (approximately 1 cubic foot) you see for sale for about $5 (a) must be a great deal or (b) what is in the bag is not what the seller claims (i.e., the good stuff). The answer is B. I occasionally find a decent bagged product, but almost NEVER at the big box stores. And, because there is no certification, the buyer is left to trust the seller and the packaging, but there is but one interest -- to get you to give them your money. Were I to institute a grading system (say 0-5) I would reserve 5 for the homemade "brown gold." The highest score I would assign the best commercially available material would be about 4.5, with most of it grading out at 3 or lower. You are usually better off choosing the stuff labeled as organic peat, composted manure or mushroom compost. The latter is livestock bedding (straw and manure) that can, but may not have been, inoculated with spores to grow mushrooms. When possible, it is best to be work the amendment into the soil.
Another major problem with the "compost" one buys is what weed seed the bag or truck load might contain -- almost all compost will contain some weeds as it rarely gets hot enough long enough (i.e., 130 degrees or more for several days) to kill the seeds within, or it was never cooked. I bought a cubic yard (27 cubic feet) of compost from a well known N Indy bulk source a few years back and am still trying to get rid of the horrible weeds it contained (e.g., cocklebur, spiny amaranth, horsenettle, nut sedge, Canada thistle, etc.). Unwanted gifts that keep on giving. Anyone familiar with farming will know that the so-called "compost" I (and other unsuspecting gardeners) purchased was instead topsoil that had been harvested from a farmyard -- likely a livestock enclosure -- along will all the noxious weeds. It was not from a fine cooked (partially decomposed) batch of leaves mixed with appropriate kitchen/table scraps and inoculated with mycorrhizal microbes. Caveat emptor. Using the aforementioned grading system, any "compost" with excessive viable weed seed would receive a zero. ADVICE: Ask about the source of the bulk "compost" AND if you see plants growing from the pile at the vendor it contains too many unwanted weeds. DO NOT put it on your garden.
Guess what top gardeners and botanical gardens prefer to use for mulch? The leaves (shredded) that so many ignorant people stuff into variously colored plastic bags in autumn and place curbside as garbage -- a practice that city government is ignorantly promoting. Finally, I suggest you would find both interesting and useful what more I have to say in Chapter 38 (Composting) of the Rantings book.