Picked up an iPad Pro to see how it compares.
It's an interesting device and its a pro device in different areas to what I would have thought. I think from an art perspective, the stylus is off the charts. I tried getting my wife to use the SP3 wiht a pro app for art and she hated it. Gave her the Apple pencil and she wouldn't put it down. I've used the Pencil a bit and I think it's incredible. I think Apple got that right and I think it's going to create some havoc in the market, just not the market we think. I think this is actually going to have a big impact on Wacom's market, not Microsoft's. The artists I know who have tried this have been blown away (do a search, they are fanboys, but not apple fanboys, most of them are wacom junkies). In short, the Apple Pencil is class leading despite the lack of eraser (which isn't a big issue because there are easy ways to deal with it). What is interesting about it that there isn't a lot of handwriting integration (no handwriting input on the keyboard) so it's definitely stylus integration aimed in a different way to Windows.
On the iPad front, it's a big tablet. There isn't much more to say. It's not a full laptop replacement no matter what Tim Cook says unless Tim's idea of the average laptop user is my mum who sends emails, browses Facebook and goes on the web. It's not going to take on the SP4's market, or at least anyone who legitimately needs a SP4. The SP4 seems to be more aimed at a single device for the power business user. The iPad Pro is only really suitable for a light business user, or the business user who has a desktop and wants some mobility in a second device. This is actually a common scenario in some companies I work with despite how annoying it sounds, particularly government. It also complements a power user in the sense that it can be used as a Wacom Cintiq style tablet for a Mac (iMac, Macbook Pro). I tested this capability using an app called Astropad and it works bloody well, so much so that I will dump my Wacom tablet and I expect this to be a big selling point for the device. I.e. if you're an artist, you can replace your cintiq (which is massive) and still have the mobility of an iPad Pro for drawing when you are out and about. Granted the Cintiq have a tablet style device to do this, but it weights more than a 15" laptop and is massive, despite being a 13".
Downsides as mentioned above. Keyboard is good (compared to the previous generation iPad keyboards) but terrible compared to the SP4 keyboard which is class leading. It's really a awkward looking design for something Apple put out. Lack of kick stand is annoying. I expect the third party cases to come up with some ideas, but I expected more of Apple.
The split screen is better than the way Windows does it in a tablet environment. It's more intuitive and easier to switch windows when you need to.
MS Office is capable but limited. I.e. it'll do the basics you need but if you need to work with macros, good luck.
Short answer - It's a good device, but not a laptop replacement as expected. It is actually a "pro" device but in different ways to the ones we would measure with the SP4 because it isn't pro in the corporate sense of the word. Microsoft probably has no reason to worry, but I suspect Wacom is going to start feeling the pinch from this on a number of levels.