I love the idea of the pen... but my work doesn't require me to use one. I've used it as a mouse from time-to-time, and I've written with it a few times just to use it. Like others have posted, I can type faster than I can write, and the result is much more easily read than my writing without a doubt. I think if I ever give up typing notes in OneNote I will set OneNote so that my "paper" is lined so that I at least have something to aim at to help keep my handwriting somewhat on track and more concise... that's my theory anyway.
All of this is probably in keeping with the way I use my SP3. I use it as a laptop virtually all the time. I don't detach the keyboard - even now I'm typing this with my SP3 propped on my crossed legs sitting on the couch. I rarely use my SP3 in tablet mode or even in portrait mode. I use the MUI apps about as much as I use the desktop though. For me, it feels more comfortable this way than trying to use it as a tablet.
On the other hand, I write on my GN3 often using its stylus on the Action Notes/S Notes. Maybe it's the size of the device that makes the difference for me - smaller and I'm more inclined to write on the device. In many ways, I use my GN3 as a tablet, reading books etc.
I use my SP3 a lot in its dock hooked to multiple monitors, all sorts of USB devices, separate BT keyboard, wireless mouse etc. In fact, reading many of your comments about the S3 and using it as more of a portable device, as a notebook or tablet, and so on has started me wondering if I would use an S3 that way rather than my SP3. Of course then I'd have 3 devices (GN3, SP3, S3) to fool with instead of just two. One the other hand, I sometimes wonder if having a backup device to my SP3 wouldn't be a good idea, but then I think of the expense involved... thoughts then circle one another like water going down a drain. All in all, sounds like first-world problems, doesn't it??