I was wondering if anyone has any experience using one of these.

How tight/loose is it supposed to be?

If it's a mechanical hackamore--short shanks, curb chain and fleece/leather nose--it can be adjusted several ways depending on the severity. According to Amy Tryon in the May 2007 issue of Practical Horseman, you should adjust the shanks so they sit about 3/4 inch from the bottom edge of his cheekbone. In addition, be careful not to position the nosepiece too low on his nose where it can press on teh soft cartilage of his nasal passages and interfere with his breathing. It should sit just above the upper edge of the soft portion of his nostrils.

While it may sound gentler, a loosely fitted curb chain can result in rougher contact because the nosepiece doesn't engage right away when you pull on the reins. This makes it harder to gauge when the contact will come into play so it's easy to overdo the rein aids. In addition, a loose hackamore shifts around the horse's nose more, making it less effective and more likely to pull the cheekpieces agains his eyes. She recommends keeping the curb snug enough that you can fit just one finger between it and the jaw. This will allow you to feel the "contact" (nosepiece engaging) immediately when you pick up the reins, so it's easier to be light with your aids.

 Hopefully someone with experience on these will reply, but from what I know ONLY a very experienced horse person should use one as they can easily hurt or damage a horse.

  I had a horse years ago that would only go in a hackamore (he's fine now, just had some soft tissue damage to his mouth from a previous owner), these are VERY severe in the wrong hands.  The other poster described the fit to a t, but I'm going to add something.  If you get a Herm Sprenger (they run about 50.00) they have a leather chin piece.  I found that adding a gel chain guard on the leather piece, made the hackamore not as severe on the horses nose, he definately was much happier.  Also, this is not one of those bits you use on a horse that doesn't turn, they also don't help with a braking problem. 

  There is a lady that use to ride where I am that used a hack and a bit, she said that way when the horse got a bit too strong, instead of taking hold of the hack, she used the bit (dr. bristol d ring).  Personally, yes I use them but usually only if the horses have had their teeth done recently and they might still b a bit sore.  I have been riding for over 20 years, but even I'm a bit cautious with hackamores, its very easy to judge your pulling a bit off and cause the horse discomfort.

  Have you tried this:

 http://www.dressageextensions.com/ProductDetail.asp?KEY=1017

I got mine on ebay for about 30 and my Paint LOVES IT!  Its thin in the middle to fit the horses palate, but a bit thicker on the sides so it doesn't slide through the mouth as much!  I love it, very soft, VERY bendy and it smells like peppermint!