Wow. Maybe it's me, but I thought it was totally noticeable. Here's a side by side...
...and here is an image from Final Fantasy: Spirits Within...
That's 2001 CGI, looking just as good as the Tarkin CGI in 2016 (actually better IMO). The same smoothness to the features. Take away the non-CGI background, and I don't see anything better about Tarkin. The only thing that makes it better than a cartoon is that there is so much static background and zero interaction with anything else. No flowing hair, etc. They did a very minimalist job to help hide the fact that it's CGI, but it is still very noticeable CGI. At least it was to me.
Maybe if I hadn't heard all the raving reviews about the CGI going in I wouldn't have had such high expectations and looking for it. I don't know, was just a major (and really the only for me) disappointment.
EDIT: I'm not saying it was bad in comparison to other CGI stuff, just that it was noticeable, and I would prefer not to have CGI people in a live action film. Takes me "out of the moment" and ruins the mood.