I tried a few. I find that they are only worth if you're either a real beginner, or you're just trying to learn a very specific technique. And even then, most of the time you can just search youtube for some good replacements, like just watching some pro draw live on Proko, does wonders for your own improvement.
Also, depending on what you are looking for, you can also try and find local classes in your area. Those are more expensive, but you'll prolly get a lot more for your money.
As for going outside and drawing, that's one of the best ways of improving anyways