I have Ultegra 6800 on one bike and can take or leave having one extra gear. Rear shifting is smooth, but so is 6700 10 speed. I'm not much a fan of the long arm design of the Shimano first gen 11 speed groups, I think the previous gen 10 speed FDs were better. I'm about to replace a 6800 FD with an 8000 because of it. I do like the ergonomics of the shifters for the most part.

If you're using Shimano and brifters, I think it's worth it to upgrade from 9 speed to 10 speed for the hidden shift cables (5700/6700/7900).
If you're on 10 speed, I think it's worth it to upgrade to 11 if you like the ergonomics and design. Otherwise, I don't see a huge benefit...maybe gearing options without using MTB parts?