I think I may have found the drawing app jackpot for the iPhone/iPad. It's called ArtStudio (Lite). The best thing about it is that it has layers! I only have the 'lite' version, so there are limits on the tools. However it is still quite capable! Here is what it looks like ---> click
Drawing Tools : The pencil tool is really smooth and quite sensitive. It's easy to adjust the size and opacity of the brush, too. I don't use the other drawing tools since they're pretty tacky. The eraser is very handy. You can change from hard to soft erasers, etc. The bucket tool is practically useless. It doesn't solidly fill.
Color Palette : You can create up to 14 swatches. The preset colors are like the ones you would find normally in MS Paint. If you double-click the swatch a color slider opens.
Layers : This is my favorite thing about this app. Since I only have the 'Lite' version, I can only create up to three layers. The layer options (Normal, Multiply, Overlay, etc) are similar to Photoshop and other drawing programs. The 'Transform' tool is my favorite. I use it to move and scale things. It's really useful and easy to use.
View Options : The view options are normal things like 'Brightness/Contrast' and 'Hue/Saturation'. There are other options, but they too are tacky.
Other : You can import and export images, resize the canvas, etc. They have built in lessons on drawing if needed. You can use the line tool, shape tools, and make the bottom layer transparent or solid white. Unfortunately, there's no curve tool.
Warning: One thing I absolutely loathe about the 'lite' version is that if the app is not open for a couple of hours, all your work is erased. Fortunately, I've found a way around this. Periodically open the app if the device is in use. If the device is not in use, leave the app open. Simple enough. Your work is also erased if you update the app.
Overall this app is super duper. It's very smooth and easy to use. No lag (except when drawing while zoomed in) and no choppy strokes. Zooming in and out is also easy and smooth.