Some are truly awful, and covered in banner ads placed to catch your hand whilst you sketch... but this one Graphite Lite, and it's sister app "pen and ink", from stepstone mobile seem well considered and executed.
The Lite version of Graphite is limited to three graphite tools, a pencil, a charcoal stick and a pointy graphite thing... all with fixed widths and sensitivity, and doesn't allow selection of some of the other "brushes" ... however these three and the 2 erasers are pretty good for basic sketching, or tracing over the imported photo layer as I've done here.
The Zoom works well, and there's a proper undo button, the only criticism I have is that I couldn't get a really tight deep gouging pencil line, and had to go over sections again and again to build up the shade, but that may be unlocked with the "upgrade"
The demo version is also limited to 2 sketches, and runs adverts in the gallery- not thankfully in the work area.
It exports to the photo gallery, dropbox, other apps, email, twitter and whatnot.
I'm plesantly surprised by the app.
Today's sketch challenge was "musical"