Feral cats can be tamed, but a lot of it depends on the individual cat. It can also take years for the cat to really become comfortable with you, and even then it probably won't be a lap cat. Most feral cats will retain at least a little bit of a wild side, though house training usually won't be too bad if the cat is fixed. Cats tend to be very clean animals when given the choice and most catch on to using a litter box quickly.
My mom has two rescued stray cats, a mother/daughter pair. The mother cat is still very reclusive after two years. She will follow my mom around, but she still doesn't like to be petted or handled in any way. As soon as there are strangers around, the cat hides. The kitten (now 2) is semi-tame and lets my mom and dad pet her, but hides from most people. Both cats were easily trained to use a litter box.
On the other hand, my aunt has an older rescued stray she found several years ago and one of her kittens (now a 8-9 year old cat). This cat has always been very friendly with people despite having no apparent owners before my aunt found her. She has an extremely calm disposition and isn't afraid of anybody. However, litter box use is sort of optional to her. She's better about it now, but a few years ago she would take a dump wherever it pleased her.
One thing that's tough about taming a feral cat is that to help them and give them medical care, you have to handle them at some point, and they don't like that. Getting the cat to trust you is tricky when all it knows is you are putting it in a cage.
