I've been using a surface pro 3 now for the past 2 years.
I got it in December 2015.

It's the i5, 256Gb, 8MB Ram version.

I use it as my main laptop and do all my development work on it.

I use the Surface Pro docking station linked to two external monitors along with a couple of external hard drives. The hub is also connected to my LAN via cable.
I also have a 128Gb micro SD card in the tablet.
For mobile work I use a Surface Pro 4 external keyboard which is great to use and a big improvement on the older Surface Pro 3 keyboard, which broke down for me recently after a couple of years of use, (for no good reason I might add!).

Apart from typical admin work using the Office suite my core work on the Surface Pro is development, primarily ASP.Net development using Visual Studio but also some Android development using Android developer studio. At any time I'd have Chrome running Inbox for my email open along with several work related tabs, Visual Studio 2017 running in debug mode, SQL Server management studio for DB work, Cute FTP, and maybe a couple of remote desktop sessions connecting to some live work servers. On top of this occasionally I'd be running Paint Shop Pro for miscellaneous image editing. All these running across two external monitors and the Pro's screen itself.

In another use case I work with video processing and use go-pro editing suite to process my videos. It's a slow process, but then again video editing usually is a slow process anyway, and is rather taxing for any laptop.

On the whole, the Surface Pro 3 performs ... OK. I love using the machine in tablet mode and really do like the whole tablet/laptop switching thing that's going on there. Sure the tablet experience isn't a scratch on my iPad, but it's still pretty good. I also enjoy using the stylus on the screen and, (when it works) it's generally a very pleasant experience. The design quality is great, and I do like the kick stand in tablet mode especially if viewing media.

So, that's the good.

On the bad side there are three core gotchas for me that basically result in me marking the Surface Pro 3 with a mediocre 5/10 rating.

For starters, this is actually my second Surface Pro 3. My original I sent back to Microsoft after a few months with it as it kept unmounting the Micro SD card, which REALLY annoyed me. Prior to sending it back I researched the issue a bit and sure enough, it's a known Surface Pro 3 issue that happens with 'some' models.
Basically you put the SD card in place and it works fine for a few hours but then, bam, it just unmounts the thing meaning you have to take the card out and put it back in again to get it working. In tablet mode you might forgive this a bit, but when docked it's a real pain having to take the machine out of the dock, reset the card, and then plug it back into the dock again and wait for it to set up.
If you google this issue you'll see most people DON'T appear to have the problem but still, a significant amount of people do experience it. For me it was just really annoying. I tried various 128Gb cards from different leading manufacturers but it was just an unstable situation with the card randomly unmounting in all cases. Microsoft replaced the Pro for me after exhausting all test cases with them. Microsoft do offer a very good replacement service which is impressive, but it's still a total time wasting exercise having to replace a work machine like this.
And low and behold, the replacement machine ended up having exactly the same issue! At this point I just resigned myself to the fact that this was a fundamental fault in Surface Pro's. So I sucked it up and worked around it, but I find this to be a very bad technical glitch, especially in a high end work laptop.

The second issue I have with this is the screen itself. ON both models I had, occasionally the touch screen stopped working correctly. What I mean by this is that you could be using it and you suddenly find a significant "dead area" of the screen where touch is not being recognised. Rebooting doesn't solve the issue. After a bit of searching on the web I found the solution is to use a Sony Touch Screen driver reset program which you run from a command prompt and the screen is returned to it's former touch glory. Now that program is on my desktop, so every time it happens (maybe once or twice a month) I just run it and it fixes itself in a minute or so, so it's that much of a hindrance, but it IS sloppy. Microsoft don't seem to recognise it as a problem but again I got this on both my original and replacement machine, and you'll see various reports about the issue online.

The third issue I have is a rather fatal issue for me and laptops. The battery. Microsoft claim a 9 hour life span. But for normal usage for me, (and by normal here I'm being kind and not talking about running in full development mode, I'm talking about maybe having some office documents open and few Chrome tabs open), I'm lucky to get about 2.5 hours. In this day and age 2.5 hours for a laptop is appalling. You should comfortably get 4.5 to 6.5 hours I think. My previous Toshiba laptop even after 3 years could still clock in a comfortable 3 hours on battery. The Surface just heats up like crazy, and the fan goes nuts as soon as you do anything at all. Yes I know this COULD be due to a rogue task in my particular setup, but I know my way around a PC and am totally comfortable with digging into the guts of Task manager and internal logs and what not. After going through 2 Pro's I can't help thinking that there’s either something going on with its power management or Windows 10 is just a general resource hogger.

So those are my gripes. On the whole, despite the negatives, I do really enjoy the Surface Pro. I think it's an ambitious machine trying to do something that Microsoft have been chasing for a long while now, the "perfect hybrid tablet/laptop". I also am a Microsoft fan in general and think they really do push things on the innovation side. I'd still look to Microsoft for my next work laptop, but would probably go back to something more Laptop in nature, namely the Microsoft Surface Book, which just looks probably like the perfect Surface Pro 3 replacement. Also Windows 10 in tablet mode, isn't really that great to be honest. It's getting there, but it's just not up to say iPad tablet standard. That's ok for me though. For me I want a laptop 80% of the time and a tablet 20% of the time. So the laptop experience is more important to me.

What are your thoughts? Do you have a Surface Pro 3, (or indeed a Surface Pro 4)? I'd love to hear about your experiences with it.