Here are some of the sketch cards I have pulled from Marvel Masterpieces Sets II and III.
I just happened to pick up a couple of packs in Boston on a whim, and pulled two sketch cards (Karen Krajenbrink's unknown screamer, and David Rabbitte's Wasp). Two cards in five packs is pretty lucky considering their 1:36 insertion ratio.
From here, I was hooked. I found my way to collect a few more sketch cards (all pulled, none bought from ebay) and was lucky enough to find a Charles Hall Elektra card.
I was also lucky to land one signature card, that being of Paul Tobin who has written some Marvel Adventures comics, the Doctor Doom and The Masters of Evil mini-series, and who was just announced as the author of the Marvel Adventures Avengers series.
The Elektra card is certainly the gem, and I may put it up for sal on eBay in the next little while!