A friend of mine has several cats and ferrets, and has had for 20 years. She was feeding her animals high-quality food (Iams for the cats - I'm not sure about the ferrets) and all of her animals had always been very healthy. Hardly any vet visits (except for vaccinations) and they were all living to ripe old ages.

Then, her husband convinced her to switch to cheaper foods to save money. She was feeding the cats Friskies (I don't know about the ferrets). Within a few months, several of her animals ended up at the vet's office - some of the cats had urinary crystals.

She said that it was because of the high magnesium content in the cheaper foods.

She has me convinced - I've always fed Max high-quality dog food, but my husband always took care of Toby and fed him Whiskas. As soon as we're out of Whiskas, I'm putting him on better cat food. He's never yet been sick, but I don't want to risk it.