I am a big fan of Pinterest, and I have something like 40 boards on my account. (You should check them out, they're actually pretty awesome.) :-D
Anyway, I knew I had pinned a link about darning knitwear, but last night I could not find it to save my life. I ended up doing a search for "mend a sweater" and mixed in with pins of Eva Mendes wearing sweaters, I found some helpful info and got to work.
The green sweater was mended with 100% cotton embroidery floss. I wouldn't say it's an invisible repair, but I don't think it looks bad for a first try.
this article on Martha Stewart dot com and this amazing sweater metamorphosis at Prick Your Finger, I got to work on the black sweater. I had a skein of alpaca/silk sock yarn that, let's be honest, will never be socks, so I grabbed it and got going. So far I have only fixed one hole, and I wasn't impressed with my darning stitches, so I went a step further and did embroidered satin stitch over the repaired hole. I think after the repairs are finished I will add some random embroidery to other spots, and make wearable art out of it.