I did two balls of t-shirt yarn, the first was a shirt that had seams, this didn't work as well because you can see the seams but it is still usable, the second was a seamless top and it worked much better, the only problem was the shirt I used has writing on it and the parts with the print didn't stretch as well.
I'll put the website link for the tutorial I used as I think it explains it much better than I have and also it has exact measurements which I didn't use as I was just experimenting and wasn't too fussed about it being so precise. I haven't made anything with it yet but as soon as I do I will write about it to say how it went.
http://www.relevedesign.com/how-to-cut-continuous-t-shirt-yarn/