I also live in the Denver area... actually about equal distance between 2 Petco's. I also occasionally stop by the local petsmart as well though it's a bit further away.
Best I can tell their animals seem to go through phases of care and neglect depending on how the employees treat them and when the summer hiring season begins/ends.
I have purchased leos from Petco on two occasions in the past. Those animals are still alive and well but getting to that point wasn't exactly fuss-free. A few vet trips and fecal tests were involved. All I can say is that a good herp vet can make quite a difference. Mine did.
Now I prefer to do my research over the internet and arrange to meet breeders at the local reptile expo that happens quarterly. I generally go with the intent to purchase a specific animal but I don't buy until I've had a chance to ask questions and look around.
I don't mind going to petco or petsmart to pick up supplies and mealworms/crickets. I do make sure to crack the lid on the mealworms and check the date to make sure they're not a bunch of dried out corpses. I gutload all my feeder insects for a while beforehand so they're in pretty good shape before they get thrown into the pit-o-doom.
Where practical, I do try to spread my visits around to some of the local "non-chain" petshops provided that they keep healthy animals. I have seen some local shops that are dismal and others that are very well tended.