I think Procreate is going to replace Autodesk Sketchbook (on SP1) for me. Its brush engine has less lag than anything I've used before (similar to Apple's Notes app) and seems to be way more advanced at the same time. You can completely modify the stock brushes or build your own from scratch using the texture library or your own bitmaps. No Pencil yet, but I'm really liking it even with a capacitive stylus.