I don’t drink milk, but I have no problem putting kind of old milk into a recipe that’s going to be cooked. Some of my recipes, in fact, want it sour.
It never smells good to me so I don’t know how I’d tell if it was bad anyway.
And mashed potatoes and boxed macaroni and cheese.
Cake? No. Pudding? No. Gravy? Yes. Chowder? Probably not.
Missed the edit window:
I was given to understand that the less fat milk has in it, the quicker it will be to go sour. That’s one reason I always buy whole milk instead of low fat or skim.
I don’t mind that much using milk that’s just starting to turn.
I don’t drink milk at all, good or bad, but for cooking I do sometimes use milk that’s a little off.
“Milk Gone Bad” would make a good documentary for elderly folks no longer interested in watching “Girls Gone Wild” videos.
It’s the opposite. The higher the fat content, the faster it will go bad once it’s opened.
I’m another who buys lactose free milk and it stays good for a crazy long time. If the Fella’s sons are going to be here for more than an hour, I supplement that with a gallon of regular milk. If there’s any leftover (which is rare), it goes home with them.
To answer the OP, once the sell by date has come, I do the sniff test too. I mostly use milk for coffee and cereal.
More technically:
Cultured “sour” dairy items (sour cream, yogurt, buttermilk, etc) are the result of lactose-digesting bacteria doing their thing.
Milk “going bad” is the result (possibly TMI) of the protein part (casein) putrefying (different kind of bacteria).
(The butterfat portion of non-nonfat milk can go rancid, but that’s much further along the timeline.)
Fresh cheeses are produced by coagulating the milk solids by a variety of processes, such as adding something acid, or rennet, or evaporative cooking.
Fresh milk is actually supposed to last a week after it’s “sell-by” date, but rarely seems to.
I’d dump anything that smells off, and buy smaller quantities if this is a regular occurance.
We buy skim milk from the supermarket and it doesn’t go sour until days after the expiration date in all but rare cases.
Occcasionally we buy milk from the corner store, and it rarely makes the expiration date without going sour.
In any case, I dump sour milk without drinking it. I usually have a carton of UHT (long life) milk in the cupboard to use if we unexpectedly run out of the fresh stuff.
I’m constantly throwing out milk which hasn’t yet reached its Use By date. I blame the supermarket because it’s always kept at the 0-4 degree temperature the 'fridge is supposed to be. I smell the milk every time I use it (coffee and cereal only) and toss it if there’s even a whiff of off-ness.
If the milk turns out to be sour…
…I’ll kill ya. 
Sour milk is vile. I toss it.
Sometimes it’s just the edge of the container that smells, though, so I’ll pour some out for the sniff test.
I wouldn’t drink it as is, but sometimes I’ve made perfectly fine hot cocoa from milk that’s starting to go off. Other times, however, heating the milk pushes it over the edge into an “off” taste that Quik cannot conceal and I end up dumping it in the sink.
I won’t drink it but will happily turn it into home made yoghurt
I am intrigued by the requesón, what is it?
Very sour is disgusting. Slightly sour is fine on cereal. Won’t hurt ya’ a bit.
Me, too. I only buy half-and-half for my coffee, and never used it fast enough before I started buying the ultra-pasteurized organic stuff.
For the OP and others who say it goes bad *before *the expiration date, I wonder if your fridges are too warm?
I wondered that, because my milk often goes off before the Use By date. So I turned my fridge down even colder and stuff started to freeze. The milk still went off before it should. I’m not convinced supermarkets store it correctly and, as mentioned further up, those open dairy sections aren’t really cold enough, in my view.
No. I don’t like milk that much, so I certainly wouldn’t drink it if it went sour. I won’t cook with it because I don’t know how it would work out in the dish.
Not just no, but HELL no. No spoiled anything. Fermented properly, yes.
ETA: With 3 kids in the house, milk never gets to even a week within the expiration date, of course. 