Marlee has a 30ft one.
AND I HATE IT THE THING!!!
I used to use it for the park, as he tends to um... "go nuts" and mostly doesn't come back until HE wants to. The park is pretty huge and enclosed, but there are busy roads outside the gates (that's why I prefer taking him to our local River park. It's far away from cars etc).
The reason I hate it: If he's not wrapping himself around tree's with it, he's getting caught up when he finds other dogs - last week, he found this other dog - who came out of know where! - and his owners got caught up in it too, they had to keep jumping it cos the two dogs wouldn't stop running around in circles.
I am now, NOT using it again.
If you can find a nice open space, like a big field or something with no trees, other people or dogs then, it can be a good idea. But I just wanted to warn you of what can happen in a local park etc.
I can laugh about it now, but at the time, it wasn't funny.
Look's like I'm going to have to work on Marlee's recall without it. Although that said, he's much better at recall then he used to be. Even better at the River park, than our local park - which is kind of annoying, as our local park is five mins walk away and the River park is about ten mins drive away - and as I don't drive I have to rely on my folks.
Have a go and see how you get on. Good luck.