I use mine as a desktop when I'm not at my desktop! haha. My Desktop has a very good graphics card and a couple TB HDDs and an SSD for my apps and Windows.
My Surface Pro 4 is my desktop replacement for when I'm not on my desktop. I have a server at home which hosts all of my documents via BT Sync. So if I need a document, I find it, right click and click "sync to this device"
That way I always have my latest files and they always sync right back to my server when I save, and then later to my desktop PC when it gets turned back on.
Another great thing is that it also keeps versions of files for as long as I want (6 months in my case).
I do some light gaming on my SP4, but I keep the resolution and details to low, that way I don't tax it.
I was debating (and still am in some ways) returning my i5 and getting an m3, but I like my device and it's working just fine. Battery life is all right though, between 4-6 hours of light usage. M3 seems to get better battery life from what I've been seeing.