Autocad who make the sketchbook pro app, have launched a new app today. I'm not quite sure where it fits in the line up. In theory because it seems to use some vector like formula, and can export at really high resolution it should be the sort of thing that is useful for preparing sketches for print. In fact the export to Dropbox option means it could usefully form part of a reasonably sensible workflow, but... To my mind It's not fully baked.
Too many random artefacts caused by the active brushes, these manifest as diagonal lines.. Sometimes these can be undone, sometimes they append themselves to strokes way back in the undo stack, and the undo is slow. Maybe on a iPad 3 you'll be ok , but stuff takes a while on my ipad2.
The killer for me is the lack of fill... You can do some sort of job using the brush but not great. Zooming in and out takes time and renders lines strangely when compared to their final output.
At £1.49... It's probably worth it for simple sketches, but it's not quite ready for anything serious.