I just got  a Dell Venue 11 (windows 8.1 tablet) and the secondary battery/keyboard

I barely use it.   In my head it was going to be great.

Debating if I want to keep it or sell it.  

What do YOU guys use your Surface/Windows 8 tablets for?

(Currently already have a laptop & an iPad mini)

This is not a Windows Vs Mac Vs Linux thread I use all three and they all have their purpose in my life.  

My and I both have the 10'' dells and enjoy them. More convenient than my LT unless I'm doing a lot of typing or spreadsheets.
I've been looking at the setup you have what don't you like?
Wife mainly plays games on hers...

For convenience I end up using my ipad.  For anything more serious I use my laptop. It just seems as an unnecessary go between

Sell it to me cheap.

Lol I'll probably end up putting it up on the EE

Let me know if you do I always need new gadgetry

I was planning on getting a Surface Pro 3 a while back and ended up with a Macbook Air.

I don't think any of the tablets are at the point where they can really replace a laptop yet.  I'm like you, iPad for certain things but some times nothing replaces a laptop.  I'm really hoping one day they have a device that does both well and I can use a single device but we aren't there yet IMO.

My wife has one...she barely uses it. She recently  bought a new Lattitude and Dell sent her a new 8" android tablet. She uses her laptop more. I have an Asus tablet that the screen isn't responding properly on, but other than that she said I could have the Dell tablet. Personally, the RT tablets haven't impressed me. I really don't like the idea of the same OS on the computer as what's on the tablet. I'd rather have a droid tablet (I'm a Mac guy but I like playing with droids in tablet form).

So what exactly is the downfall of the Surface Pro 3?

Here are the factors that mattered the most for me.  Lack of a regular SD slot on the Surface is a problem for me when I want to upload pics from the camera.   If the cost of the Surface included the keyboard then it would be a closer contest but having to spend an additional $125 on keyboard is a deal killer as well.  The multiple desktop feature of the Mac OS also benefits my productivity on a 13" screen.  

I've love to pick a Surface up cheap to play with and see what I think of it when using it daily but I couldn't convince myself to spend that kind of money on something I wasn't sure would really work for me.  

Everyone's needs are different so for some people it will probably work pretty well for them.