As actors, Smith and Coleman bounced off each other wonderfully. Their characters had a rather whimsical, innocent friendship about them. The problem was that Clara was an enigma before she was a character. She was a story arc, a riddle for the Doctor to solve. Their chemistry was great but held back by the story, and indeed the quality of the writing itself. The character of Oswin had proven I had no problem with Jenna, and I knew already that I liked Eleven... but it wasn't until around the time of the 50th anniversary episode, coincidentally just after her 'impossible girl' story was resolved that I really began to enjoy their friendship. Unfortunately, it ended in the episode after that.
For me, Capaldi and Coleman go to show perfectly how age really doesn't matter. I loved Nine and Rose, I loved Ten and Donna - but in both cases it was largely down to me really liking one or the other and them simply bouncing well off their counterpart (Nine and Donna respectively). Twelve and Clara is the first duo where I've really, really, really liked both leads and their chemistry. They could have gone the whole parent or grandparent route with such an age difference, but instead they work fantastically as just friends. They have a very close friendship that I buy means the world to them both. It's going to make Clara's exit a tough one for me, personally.