A crate divides opinion because it can be either of two very different things. Introduced with a little patience it becomes a den a puppy chooses for itself; used as somewhere to shut a dog away, it becomes a punishment. The equipment is the same; the training is what decides which one you get.
Make it somewhere good to be
The whole game is teaching the puppy that the crate is its own safe spot. Leave the door open at first and let it explore in its own time, feed its meals inside, drop treats in for it to find, and give it comfortable bedding. Never use the crate to punish — a dog that is put there in anger stops trusting it, and the safe place is gone.
Building up the time
Once the puppy goes in happily, start closing the door for a few seconds and opening it before any fuss, then stretch the intervals slowly. Sit nearby before you try leaving the room, and keep early sessions short and calm. The pace that works is the puppy's, not the calendar's; pushing a full night too soon is how the association sours.
Where it goes wrong
A crate is a bed, not a holding pen for the working day. A young puppy cannot hold its bladder for long, so hours shut in guarantees an accident and a distressed dog, and no crate substitutes for exercise, company and time out of it. Whining that does not settle, or real panic at the door closing, is a sign to slow down rather than to wait it out.
Is it cruel to crate a puppy?
Not when it is done as training rather than confinement — a properly introduced crate becomes a den the puppy likes. It turns cruel when it is used for punishment or for long hours that a young dog cannot cope with.
How long can a puppy be left in a crate?
Not long. A young puppy cannot hold its bladder for more than a couple of hours, and a crate is never a substitute for exercise and company. Long stints shut in cause accidents and distress; build up the time slowly and keep it short.
How do I get my puppy to like its crate?
Leave the door open, feed meals inside, toss in treats and add soft bedding, so it forms only good associations. Close the door for seconds at first and lengthen it gradually, and never use the crate as a punishment.