My SP3 arrived at the weekend, and has proven interesting to compare to the SP1/2 earlier models.
It is bigger, lighter, thinner, and has a higher resolution screen:
The weight difference is surprisingly noticeable. It is distinctly lighter, and thinner, and you appreciate that immediately when handling, compared to the SP1/2 form. This is a definite improvement. I also like the new color change: brushed Alu rather than Black.
The increased size (in width x height) is a personal preference if that is a plus or minus. I think I prefer the form of the original, but perhaps it just takes time to adapt from familiar to new. The bezel - the black area around screen that shows nothing - has shrunk a bit, so some of the extra screen size has come from that rather than dimensions. I wonder if we may see a 'Pro Mini' version with much smaller size - like iPad and iPad Mini.
The screen resolution of 2160x1440 (vs 1920x1080 on SP1/2) is useful for showing more of your image, but is not a meaningful change of button/touch size given the extra screen real estate it's spread over. Brightness is about the same, or close to.
Some buttons have moved around, and the USB3 port has swapped sides. I would have liked a lockable power button as its still too easy to turn it on by accident/in the case, but that didn't happen. Some of the accessories have changed size/ weight of course - the type cover (there is no touchcover option anymore) is bigger and heavier and backlit, but the charger unit is smaller and lighter, and has a much better connector.
It's early days yet, but I think I'll keep it.
Oh and, while its getting good reviews mostly, it is apparently impossible to repair or upgrade. Everything is glued in, so don't think you can access it to swap processors or put a new battery / extra memory in there.