Back when I used to draw a weekly webcomic with my boys I briefly dallied with Autodesk Sketchbook (desktop), but reverted to my favourite lightweight (and cheap) Photoshop Elements.
What's interesting is that for some tasks I find it far better to have a limited set of tools. I can do almost everything I need from a desktop drawing app in Photoshop elements before my Creative Cloud version of full fat photoshop has even loaded.
Conversely on the iPad many of the drawing apps seem to offer just too little functionality to be used in any form of serious workflow. An exception to this is Autodesk Sketchbook pro, which has been a solid performer right from the start, and in a way that doesn't force me to substantially change the way I work.
It offers layers - 18 layers if you can restrict your image to 1024x768, but can even handle 3 layers at 8 mega pixels- that's 2830x2830.
The puck that controls brush properties gives a really speedy way to change brush size, by selecting and sliding left for smaller, right for larger, and down (and up) to change the opacity.
There's even a built in virtual palm rest that tracks whatever touches it, so you can steady your hand.
Undo is a simple back arrow button, there's a time lapse record function so you can show your working, and there's even a "Duo Sketch" function if you want to sketch a long with someone (I never have)
...and best of all there's a really simple flood fill function... a bucket and a bunch of COPIC marker colours to pick from.
Finished items can be exported to the photo gallery, and also emailed as a .psd.
...and just for fun it looks like this version has been deprecated... I can't find it in the appstore at all.
There is a cut down Sketchbook Express for iPad, but it looks like all the effort is being put into a new version: Autodesk Sketchbook... which I'll take a look at sometime in the coming month... but on first glance it offers a very basic set of tools for free, a few more tools for "essentials" when you sign up, and some pro tools for a subscription.
This does highlight an issue for those planning to use a tool for serious work, the update cycle of iOS devices seems to force a swift iteration on app versions. Good for getting new features... terrible for messing up workflows.
Oh, nearly forgot.. the sketch a day challenge for today was to draw a pencil... which is apt as that's the new logo for Autodesk sketchbook.