I am new here
but I thought I would just jump right in and join the discussion. I am a lover of the crate training, provided as was stated above it is used properly.
A crate in my opinion, should become a safe haven for your pup, and then grown pet. Of course they don't adapt overnite to one, but with time, patience, and making it a positive experience for them, you can turn it into a place they love to be when they have to.
I have a rescued boxer, that does suffer from separation anxiety. She was 2 yrs of age when we got her, and we immediatly started training her to use the crate. It was important to her safety that we do this, as she tended to get wild when we left, and causing injury to herself. I used treats, toys and praise to get her to adjust. I started out by coaxing her in there with a fav. toy, from there we would put a treat in there for her to go in and get, so she would learn it wouldn't hurt her, then we started leaving her crated for short durations while we went out, gradually increasing the time. Kailee now loves her crate, and goes to it when she seems to need some quiet time.
One good tool I have found for crating is the Kong, I stuff it full of treats before we go out, thus distracting her from our actual departure, and by the time she gets it cleaned out she is usually tired and falls asleep. Here are some instructions I followed while crate training.
Take a tasty treat and / or toy. Lure the puppy into the crate, praise and make a general big fuss with the puppy while they are in the crate, give the treat, have a little game with the toy and then allow the puppy to come out of the crate. Do this 2 to 3 time in a row, several times a day. Your puppy will soon be running into the crate of his own accord, so you can now put a 'word' to the crate for your puppy. 'Crate' 'Den' or 'Bed' are quite good words to use.
Once your puppy is happy in their crate, close the door and leave them in there for a couple of minutes. If you do this when your puppy is tired in all they may even lay down for a sleep when you close the door. Keep repeating this, letting the time your puppy spends in the crate build up. If the puppy cries ignore him. Only open the door when the puppy is quiet and calm. If you do comfort the puppy or let him out of the crate if he is letting his displeasure show, he will be very quick to work out that this type of behaviour works to get what he wants.
You will soon be able to extend the time your puppy is in the crate. Many pups and adult dogs will retreat to their crate (or 'den' to their way of thinking) if they want some quiet time. The crate should be your pups own personal space, just for them, somewhere quiet but where they can still see what the rest of their pack (you and your family) are doing.
Never leave a young pup or adult dog in a crate for long periods of time unless absolutely necessary. The crate is just a training aid and safe house for your pup and should not become a total way of life. Young pups should be crated or denned for their own safety when they can not be supervised, but they need to come out frequently to go to the toilet - even at night. See the article on housetraining on how to manage calls of nature with your puppy.
As your puppy get older and more sensible (this is possible with a Boxer LOL), you will have to crate him less and less. You may find that you go several days without using your crate, but it is still a good idea to have your Boxer go into his crate occasionally so he is happy with it when you do need it. Many people leave the crates up all the time with the door permanently open and their dogs can go in and out of them as they please.
Kailee: Female Brindle Boxer. The Light of our Lives. Adopted by us October 27th, 2001.
Buddy: Waiting at the Bridge. Flashy Fawn Male Boxer. May 4, 1993 to February 21, 2001