Line is the "precision drawing" app; I put it in quotation marks because it's really just less of a freehand tool than Sketch. The precision aspect comes from the Trace and Stamp packs and the perspective grid. I'd probably call it a technical sketching app.
Line organizes your work into a gallery of drawings, which you can name, duplicate and share the same as with the Pen Tip menu. (Sketch is a little different.) In the drawing view, a toolbar along the bottom supplies six drawing tools -- two pencils, two fine-point markers (0.25mm and 0.5mm), a brush and a fat-tip marker -- plus an eraser.
There are six color swatches that you can either pull from Kuler or via color picker, and an icon that pulls up a perspective grid. At the top sits the Touch Slide toggle, the Slide and Trace Packs menu and the pen connection menu. You can also expand into a toolbar-free full-screen mode.
Long-pressing the brushes bring up options for size, opacity and blending; blend off overlays the next brush stroke, but with it on, it seems to perform more of a luminance/hue overlay than a color mix. You can pull in a photo for tracing from either the Camera Roll or your CC files.
I do love the Trace packs -- I have pretty pathetic drawing skills, and the ability to create accurate polygons is really useful. (I'm a polygon doodler.)Tapping on the right Touch Slide control cycles through various polygon shapes. Snap guides pop up for intersection, right angles, parallel lines and so on as you move Touch Slide around the drawing.
In contrast, though, Concepts has a grid and the ability to stroke custom curves; more notable, the Pro version offers Spline editing.
The strokes certainly look and feel pretty natural to draw, both with and without the stylus. Only the brush seems to be pressure sensitive, responding in both density and size, and you have to press quite hard to get the tablet to sense the stroke.
Sketch seems less of a standalone creation tool than an adjunct to Behance. Here, the app organizes your work into Projects, each of which is composed of five sketches by default, though you can add more. That allows you to create variations of a sketch that you can upload as a group to solicit feedback. You can reorganize the sketches within the group, and interact with feedback directly from within the app.
The drawing screen is similar to that of Line, except a more abbreviated toolbar sits along the top. It only has five drawing tools -- a pencil, a fine-point marker (which looks like the 0.25mm), a brush and a fat-tip marker -- plus an eraser. It only shows you the currently selected color, but pressing it brings up access to Kuler colors, a color picker, colors pulled from an underlying image, and a handy color history.
Next to that sits the Share menu, and an icon to load an image -- in Sketch you can pull in from the tablet camera as well as the Camera Roll and your CC files. Unlike Line, the Slide Touch Slide only supports basic shapes -- line, circle, square and triangle -- and photos can be used for collage, not just tracing. You can also expand into a toolbar-free full-screen mode.
The brush and fat marker behave differently in Sketch than in Line. There's more color mixing, the app seems more sensitive to light pressure, and the marker shows increased density when a stroke overlays itself. While Line and Sketch are differentiated by function and color, the interfaces are also slightly different for no apparent reason and in ways that can be annoying if you use them both. For instance, the toolbar runs along the top in Sketch and the bottom in Line.
I also find it bewildering that Line lets you adjust size, opacity and blending settings for brushes and markers, but Sketch doesn't. It would be really useful to be able to, say, at least set a minimum brush size or maximum opacity. I understand that we don't want a single bloated app, but some of the differences seem arbitrary.
There are a couple of potential annoyances with both apps. First, for best operation you have to turn off multifinger gestures in the iOS settings. That's not Adobe's fault -- there's no way, at least in iOS 7, to turn it off selectively within an app. It also decided to run a backup when I launched the app without offering any way to cancel or reschedule.
More problematic, you can't use the apps unless you're logged in to Creative Cloud. Yes, they work offline, but only if you logged into CC before you went offline. Given that CC logs you out after a given period of time, this has the potential to be incredibly annoying for infrequent users. And as I mentioned earlier, I couldn't reliably access the Cloud Clipboard.
Is it worth it?
Here's the thing with the Ink and Slide bundle: essentially, you're just paying $125 for a nice stylus. In contrast, the Jot Touch costs about $100 (£80, about AU$145) and the Wacom Creative Stylus 2 only $80 (£65, AU$100). Granted, Ink is a really nice stylus, but all of the Creative Cloud-related features are replicated in the apps. It really needs to come with, say, a free year of the Photography subscription or have a discounted price for current CC subscribers -- something to make it feel less expensive.
Unless you're a current CC subscriber, the apps are handy but not a terrifically compelling option. The unique and interesting features -- syncing with Photoshop or Illustrator or sharing on Behance -- only make sense if you've got the paid CC subscription, since it requires the CC versions of the applications. A small group of users will probably find that syncing to be the feature they've been waiting for in order to justify subscribing. As simply standalone apps, however, there are comparable or better choices available already.