I really do love this thing.
I thought I would take Mark's suggestion, and offer thoughts from the perspective of my particular expertise. I'm a pianist and a composer. I write music for film and television, and I use a pretty souped up Mac to do it. I've been playing the piano from a young age, with classical training. So, although my job doesn't necessarily call for a lot of typing in and of itself, I am a professional hand user, as it were. Because my composing rig isn't really portable, though, and I don't have a laptop, I was excited to use the TextBlade to tackle my voluminous correspondance while away from my desk, with my phone. I often go to a coffee shop, or am travelling, and I wished I could have a more desktop typing experience in places like that. (Right now, I'm typing this in the lobby of the W Hotel on Hollywood Boulevard, albeit for the very unglamorous reason that I'm waiting while my car is being serviced a couple blocks away.)
On that score, the TextBlade delivers just beautifully. I would echo all the great things so many other people have said about its feel, its comfort, and its useability. The build quality is palable, and very satisfying. It feels very cool to essentially turn whatever table I'm sitting at into a laptop computer, with keys wherever I want to put my hands, and my phone tilted up, at a very comfortable angle in the NanoStand, floating free at whatever viewing distance I please.
[Someone just came over to my table to ogle my Textblade, and was extremely impressed as I eagerly showed it off.]
What I notice about my hands as I type on the TextBlade is that they stay in what I would call proper piano-playing position, significantly moreso than with a standard keyboard. I remember my piano teacher scolding me to keep my fingers curved over the keys, without letting them splay out flat, so that I would strike the piano keys more with the ends of my fingertips than with the flat part opposite my fingernails. As I studied more, I learned more about the benefits of this technique. Ergonomically, it is what is most comfortable and natural for your muscles and tendons, enabling them to play fast passages for long stretches without fatigue. From a technical perspective, this position also affords the player the most power and precision. You can put more strength into the notes that way, but you can also enjoy more control over a lighter touch.
The way my hands naturally interact with the TextBlade, I feel like I'm playing an easy, comforable C major scale, and making my teacher very happy about the way I'm using my hands to do so. The closer vertical spacing of the letters, compared to a standard keyboard, actually helps to keep my hands in form; when I went back to using my full-size keyboard, I was actually surprised to notice how far I had to move my fingers, and how outstretched they became.
Because of this, as well as the wonderful placement of cursor movement and editing controls under the home row keys, I would say that the TextBlade really rewards proper touch-typing technique, with your hands in home row position. When you put your hands there, they feel really ensconced there. I would say, though, that for applications that don't call for your hands to be so ensconced - for example, my music creation software, for which my hands are constantly darting around between a mouse, a mixing board, a piano keyboard, and specific individual key shortcut commands - the TextBlade is a bit harder to use than my full-sized keyboard, with its chicklet keys. If my hands aren't already in typing position, it's easier to jump in and find and hit the V key to toggle track automation view, for example, on the full-size. Because of this, I don't think I'll throw it over entirely for the TextBlade.
But for extended typing, for portability, for coolness, the TextBlade is truly awesome.