GSD puppies Rescue
German Shepherds and Saint Bernards have fairly different temperaments. Having said that, there is greater variation within a breed than between somewhat similar breeds. GSD/SB mixes could be totally like a German Shepherd, totally like a Saint Bernard, or anything in between. What is more indicative than breed type, is what the parents are like. Are the parents aggressive? Friendly? Hyper? Odds are, the offspring will be like the parents. However, I'm sure you know siblings who share both parents, who are nothing alike. The same goes for dogs. Littermates that share the same mother and father, can be totally different.
As Patricia Shoup answered, these dogs will shed. A lot. Plan on becoming very good friends with your vacuum cleaner. And yes, they will be expensive. Everything about large dogs is more expensive than smaller dogs: they eat more, they require more anesthesia so surgeries cost more, they require larger doses of medication so medication costs more.
It's a myth that mixes are automatically healthier than purebreds. What Mendel found is that F1 hybrids (first generation crosses of pure strains) combined traits of both, and these hybrids (at least the ones that sprouted and didn't die right away, because Mendel was working with peas) faired better than the pure strains. But when you cross a hip dysplastic German Shepherd with a hip dysplastic Saint Bernard, you're going to have hip dysplastic puppies. (at least the chance is quite high) If you cross an aggressive doberman with an aggressive Chihuahua, you'll have an aggressive doberhuahua. If you cross a dysplastic German Shepherd with a Cocker Spaniel with hereditary heart problems, you will have puppies with a high likelihood of having hip dysplasia, heart problems, or both.
So, if you're thinking about getting a puppy, look at its parents (or other close relatives) for an idea of the temperament and health. The puppy will MOST LIKELY (but not 100% certainly) be some mix of those traits and characteristics. And don't forget that there are tons of health problems that can't be detected just by looking, or that may not manifest themselves until the dog is considerably older (like degenerative myelopathy, for example, which usually doesn't affect a dog until it's 8 or so, but which will cripple a dog and leave them unable to walk.)