Iams is not a "horrid" food. It's not as good as it could be, but it's also not as bad as many others out there. It contains chicken and chicken meal, which puts it at mid-range despite the other undesirable ingredients.
OP...the other brands you mention are all good foods and I do second the suggestion of the grain free varieties. Earthborn is probably the best value for ingredients vs cost. You might also want to take a look at Taste of the Wild.
Im not sure what ingredient list your looking at but this is it:
Iams ProActive Health Mature Adult Hairball Care Cat Food
Chicken By-Product Meal, Corn Grits, Dried Beet Pulp, Powdered Cellulose, Ground Whole Grain Sorghum, Corn Meal, Chicken, Animal Fat (preserved with mixed Tocopherols, a source of Vitamin E), Dried Egg Product, Natural Flavor, Potassium Chloride, Fish Oil (preserved with mixed Tocopherols, a source of Vitamin E), DL-Methionine, Brewers Dried Yeast, Vitamins (Vitamin E Supplement, Vitamin A Acetate, Niacin, Ascorbic Acid, Calcium Pantothenate, Biotin, Thiamine Mononitrate (source of vitamin B1), Pyridoxine Hydrochloride (source of vitamin B6), Vitamin B12 Supplement, Vitamin D3 Supplement, Riboflavin Supplement (source of vitamin B2), Inositol, Folic Acid), Choline Chloride, Salt, Minerals (Zinc Oxide, Manganese Sulfate, Copper Sulfate, Potassium Iodide, Cobalt Carbonate), Rosemary Extract
Unless you are counting chicken by product meal as good.... its pretty bad.. corn grits are second... Meaning its probably more corn than anything else... = horrid.... May not be as bad as meow mix etc... but still pretty bad... I would defiantly not put it mid range in my opinion... Iams Healthy Naturals has chicken and chicken meal... That might be put a little higher but it still has byproducts second..