You can find a lot of technical information on the Internet about hide glue; here i just want to describe the use i make of it and share some practical thoughts and tips about it.
I think the most important thing with this product is to get the feel for it, and to do that without risking to compromise your instruments, you should do tests; this don't necessarily need to be scientific, instead i much more prefer an empirical approach. It's a lot like cooking.
First of all, not all glues are made equal, and even different batches of the same product come out different; so instead of relying on label information, it is much better to learn how to test it for quality. Also, it is not pure strength what you are looking for, since the average hide glue is stronger than what you need in a guitar making application; is flexibility instead what you need, since the wood is not at all dimensionally stable, more so in a vibrating instrument, so the glue have to accommodate with all this, without breaking and without letting the joint slide (as synthetic yellow glues do).
For a strength test, just glue some parts together, for example a bad top you are never going to use or similar stuff; cut strips perpendicular to the joint and break them apart; if the wood breaks and not the joint, it's strong enough (that is, if your top is going to crack, is going to do that anywhere but in the joint!).
For a flexibility test, prepare some hot glue, put a big drop on a piece of paper and make it drip; after a day or two, when it's completely dry, bend it and if it breaks easily is not good. Usually hide glue can be bent, while bone glue breaks, so of course hide glue is what you are looking for.
Another easy test is for checking thickness; many beginners ask for water to glue ratios. Well, depending on what you want to do, you use different ratios, but again, every batch of glue or even every spoon from the same bag dilutes a little differently. A basic starting point can be from 1 to 1 in volume, to 1 glue to 2 water, again in volume. Then you let the glue soak up the water, and eventually warm it up to working temperature. When it's ready, put a big drop on a piece of wood and check how long it takes for it to gel, touching it every 30 seconds or so. I like it to stay liquid for at least 1 minute or 2; since i warm up the pieces i glue, i will have even more time when I'll actually use it, and that's plenty for most applications. Also, try thinning it even more, and do the strength test again, to make sure you're safe.
A big advantage of hide glue over any other i know of, is that it is reversible; this means that you can take a joint apart applying heat and moisture; also, if you glue something and don't like the result or you messed up something, just remove the part, clean it with warm water, and you can redo the process (better if you let the parts dry up again before). Repairs, that have to be reversible, have to be carried out with hot glue; it's a shame that way too many 'repair guys' don't even know how to use it. In this respect much can be learned from the violin world (if someone would dare repairing a Strad's crack with epoxy or super-glue or whatever other than hot glue, he would most probably get sued and end up having to pay much more money than he could ever afford to).
A nice consequence of reversibility is that even if the glue cools down and thickens, as long as it does not dry, that takes hours, it can be re-heated. Every beginner gets scared about gluing a back, because it takes quite a bit of time to get the glue on the whole perimeter of the guitar, and some more time to get pressure on it, whatever method you use. For this reason, so many think white or yellow glue to be better, because of the longer working time. With the right technique, you have longer working time with hide glue. In the case of gluing a back, apply your glue on the rim, put your back on, put pressure on it (traditionally with a string) and then re-heat the perimeter; that simple and very effective. A similar technique is traditionally used to glue bindings, too.
A common complaint is the fact that it is not ready to go; you have to mix it in advance, heat it up, take care not to overheat it, and it does have a pretty short shelf life once mixed. Well, just get used to it, get in the habit of thinking in advance when you'll need it, and you'll see that it will be as easy as using any other glue. You don't even need a 'professional' glue pot; i personally use a baby food heater; it's very cheap, it has a thermostat, and it's made to use with baby food jars; just get a few of your favorite flavor, eat it, and you'll have your glue pots. To control the temperature, i use a meat thermometer.