Microsoft Surface Pro 4 - It's not my primary machine and that's just fine
So I'm a Windows guy. I can use anything but I just prefer Windows. I like running things from a Command prompt. I like trying software that has no business running on my machine. Oh, and like connecting to my office servers the way they were meant to be connected to. In general, I just like to dig into the OS and work things out on my own. So I really like a portable device that gives me Windows. There is a place for an iPad, there is a place for an Andriod tablet, but in my day-to-day, I love the Surface Pro 4.
I'm going to assume you've compared all the devices you are looking at for weight, screen size, memory and hard drive space, etc. I'll try and stick to the usability pros and cons and try not to sell it too hard (but I really am enjoying it).
First of all, the stylus, or the pen, is great. I moved from the Surface 3 to the 4 and the differences are amazing. Best one, the eraser. If you are using a stylus without an eraser than I guarantee you are one of those people saying "its just not like writing with paper and a pencil". Well, it can be. I use that eraser ALL THE TIME. In OneNote, in Nebo (great note taking app), in Evernote, etc. I use the pen so much more effectively than with the Surface 3 that I really was surprised (I rarely use the onscreen keyboard anymore).
The other feature I love, the face recognition for logging in (Window Hello). I try to be secure so I have the screen lock pretty quickly and it's so nice to just have to glance at the screen instead of typing my complex password 30 times a day.
Beyond those two new Surface 4 features, the device is a great windows machine. The Microsoft ecosystem is obviously pretty robust and my office uses Office 365 so all the Office apps are well connected and synced to my main desktop. I installed Chrome on the Surface so all my Google apps are synced across devices. Using Chrome Remote is smooth as silk from the Surface to my home and office machines, my Mighty Text app (check it out) shows all my phone information, etc.
I also can manage all my work from the Surface if needed. I work on a dual screen monitor at my desk with full versions of everything but in meetings, with the Surface, I constantly pull up PDF's people are referring to, websites to show what I'm describing, and live data from servers, databases, and anything my desktop would access. Granted I could do this from any portable windows computer but the tablet form factor, the easily removable keyboard, and the gorgeous screen almost make it seem like I'm showing off when I do it with the Surface (and as a Director of Technology, it's good to show how I'm controlling the tech and not the other way around.
Of course there are some trade offs for any slim portable device (with a Surface Pro they are proprietary power adapter, HD space, limited monitor size, flimsy keyboard, the inevitable slowness over time as apps get more hungry and new devices get more powerful) but we have people in the office using these (and Surface 3s) as their main machines with docking stations and dual monitors and they all seem to love them. So don't replace your computer yet but if you can afford a second computer, pick up a Surface Pro 4.