I just finished my first project with reclaimed yarn, it wasn't the easiest thing to unravel the sweater but now that I have my shawl finished it is so worth it.
I used the tutorial on this page as a guide. It's fantastic! http://www.neauveau.com/recycledyarn.html
Here's another one that helped, although I didn't get nearly as much yarn from one sweater as she did. http://www.az.com/~andrade/knit/thrifty.html
The sweater I bought was commercially made and I checked the seams before I bought it. The strand used to stitch it together was the exact same color but slightly thinner. I didn't worry too much about keeping that intact when I was unraveling. Each piece of the sweater yielded one long strand of yarn. I balled them up piece by piece so that I had 5 balls(1 front, 1 back, 2 sleeves, 1 collar).
I made a single crochet triangle shawl out of my yarn and it's fairly large. I haven't measured it, but I'm plus sized and it covers me well.( I'll post a picture of it in the finished section when we finish the roll of film the pic is on.)
I hope some of this helps.