I bought one of these before starting potty training, I would have been too scared to go out without it as to start with they cant hold on for long.
THEY ARE EXCELLENT. The wee/poo stays in the bag no leaks and no smell and can be disposed of when you get a chance, no matter where I am if he needs the loo I am prepared! Be it the park, legoland (which we went to just after we started potty training) the beach or the car - I can pull over (when safe to obviously) and let him go to the toilet.
I cant recommend this enough to anyone. Its not the most comfortable but he now stands up to do a wee into it....
One down side and that is the cost of the liners.. but think overall worth it.