I got mine the other day. So far... I LIKE IT!
I went through each of the nibs to see which one I liked the most. Right now the default one (HB) is what I'm liking the most. One thing, if you've got any experience with real drafting pencils, and know how each of the hardness in real life, you will notice that the nibs really don't match. I was really looking forward to the B hardness, as it's actually my real life personal favorite. But the B nib isn't even close to a real B lead pencil.
I am really enjoying the flip to eraser. It's much more natural for me. With the old pen, I was always having to stop, rotate the pen in hand to find the eraser button, click then erase. Now I just flip, wipe, flip back, write. Also with the flat all along the side of the pen, I naturally, without thought, hold the pen with my index finger on the flat, with corresponds to the select button.
The magnet on the pen with the SP3... meh... There isn't enough magnetic material around the edges of the SP3 to properly secure the pen. There is the volume rocker on the left side, but it only will line up with one side of the pen, so the other end flops around. The same with charge port on the right side. There isn't anything at the top for the magnet to attracted to. The bottom (keyboard connector side), same issue with the sides. So in summary, with the SP3 you keep the pen loop to secure the pen.
Summary: So was it worth the $$? For me, yes. I use the pen quite a bit for handwritten notes in OneNote. It has improved the legibility of my handwriting over the old pen. I just wish the nibs were closer to the real thing for friction, but it is a nice improvement over the old pen. The eraser on the end, definite improvement. The securing magnets on the SP3 just doesn't work, but it's not the pens fault, the SP3 just doesn't have enough internal magnets to make it work.
BTW, I'm still running W8.1 and the pen works just fine with the SP3.