Do you ever wonder if you'll ever be able to have a dog that can be trusted off-leash? . . . First let me cover a few points here . . .
I am not a huge advocate of dogs being off-leash due to safety issues and other dogs, i.e., dogs I don't trust or that are nasty. Not only that, there are leash laws in just about all cities now so it's usually against the law no matter how well trained your dog is or how well he may listen to you. A leash law is a leash law.
Ok, however I do believe that a dog should be able to be trusted off-leash if need be or if you are in a safe place and they are allowed to be off-leash. I often get the question right off the bat--"Will my dog ever be able to be off-leash and trusted?" The answer is "Yes." The solution is training, lots of it.
If you have a good relationship with your dog, you've done lots of training, classes and your dog listens to you then you will very easily be able to have your dog trained off-leash. Of course dogs will be dogs and a squirrel, cat or tennis ball may be much more fun than you but with the right training and consequences when they do run off during a recall, your dog will be easy to manage off-leash.
You can read many "breed books", of which I put in quotes because I hate breed books, that state that Jack Russell Terriers are not good off-leash and often can't be trusted off-leash. Well my Jack Russell is amazing off-leash and I can call him away from just about anything. He's been mostly off-leash during flyball training and practices but some of that was in a wide open park. It can be done, no matter what you may read or hear . . . it's all about the training and relationship you have with your dog, not the breed.