I got an Adonit Jot Touch for a birthday present this year and I've been working with it on and off. Like many people, I've been waiting for a good stylus for the iPad that has pressure sensitivity, and this seemed to be the one.
The most important conclusion I've come to is that the software is key.
I was initially really disappointed. Using the Adonit with Artrage, or with Sketchbook Pro was very laggy. When drawing a line I often had over a centimeter between where the stylus was, and where the end of the line was on the screen. And this wasn't due to me drawing particularly fast. That kind of lag makes this hard to use for quick sketching - the exact task the iPad should be great for. When I switched to my finger, the response time was incalculably better. The experience of drawing with a finger in Artrage or Sketchbook Pro was nicer than with a stylus - not exactly what I was hoping for.
However - keen to identify whether this was a fault of the software or a barrier thrown up by the bluetooth communication time - I opened up Procreate. In Procreate the lag is hardly noticeable - which makes me very happy indeed. If Procreate can get it working, then Artrage and Sketchbook should be able to solve this too. I love the features in Artrage, and would much prefer it to Procreate for working up sketches.
With the lag addressed - what's the rest of the pen like? The clear plastic disk attached to the tip make it very easy to see where you're drawing (though if you pick one up make sure to add a screen protector to your iPad screen). The pen has a lovely weight to it - and the magnetic usb charger is great. It holds it's charge for a long time - it's sat in my bag for months without being recharged. Pairing with the iPad is also easy - all the programs I used recognised the pen quickly and 'just worked'.
I actually like using this pen in Paper - though of course that app has no pressure sensitivity. The precision alone is a good feature.
But what about the pressure sensitivity? I have to say I like it a lot. All three of the apps I tried it with have pens that have a width sensitivity to the pen pressure (or something that mimics it closely). That makes creating ink line art feel much more natural and sketchy than a non-pressure sensitive stylus. The colouring with opacity set to pen pressure is generally a little more clunky. Procreate overrides the opacity and ties it only to pressure. So you can't have a maximum opacity of 10% at full pressure for example, so you're confined to just pressing very very lightly if you want low opacity brush strokes. However, these are software issues that are likely to evolve with future releases.
Conclusion? I like the pen, it feels natural and it's night and day better than drawing with a finger on the iPad. Is it worth stumping for pressure sensitivity? Well, the software doesn't feel like it's quite there yet. I'm an optimist, so I believe that Artrage and Sketchbook Pro will fix their lag issues, and then the more powerful features of these will be brought to bear. If you're looking for a Cintiq replacement - look elsewhere. There's no full Photoshop clone on the iPad and you'll never have the same control over the degrees of pressure sensitivity and how they map to brushes. But for quick and dirty sketching - I think this is a nifty tool that will only get better as the software catches up.
I'm using the Adonit Jot Touch with a 32Gb iPad 2