The answer is it depends.
For me I'm fortunate enough to afford all consoles. So to pay for an Xbox to play Halo/Gears/Forza - three series I enjoy - it's worth it to me.
Not to mention there are quite a few features to the XB platform that I prefer over Sony's; external Hard Drive storage capacity a HUGE one as well as I prefer it's controller over the DS4 for certain games, shooters a big one obviously. These have led me to prefer it for a big number of multiplatform games as well.
So yes, I think it's very much a viable platform still.
Not to mention that as of this point, I have more faith in them having FC/BC currently and going forward than Sony.
Now if you are asking would it be viable if you could only afford one console? That's a different story, then it would all come down to do you prefer those three exclusives + multiplats over Nintendo (I'm not buying that the Switch will be an adequate multiplat machine) or those three exclusives over all of what you would get with Sony?
For me I would not buy an Xbox if I could only afford one console, but for some I could see that those three plus the multiplats being enough.
Would it be viable for a large portion of NeoGAF? No.
Would it viable for the average US and UK consumer? Maybe.
I would hope Xbox wants to expand beyond those two markets though.
This is actually the best answer to the thread question IMO. Except I would change that it would be viable for the average US and UK consumer to "Yes." (It may not be the "best" choice, but it would be a viable one esp if the price was right.)