I am shocked that Adobe felt this feature was release-ready way back when, and has never fixed it.
1. It misses at least 10% of all faces, even when they are large and clear in the center of the frame, but it does get birds, shrubbery, clocks, and flowers (a few of the items it has tagged as faces for me). Missing 10% of all faces makes the feature completely unusuable right off the bat. It's a non-starter. You're just wasting your time and endangering the integrity of all of your work.
2. It's absurdly slow
3. It fails to assimilate faces that are very obviously the same
4. It fails miserably to accomodate exisitng keywords and hierarchies
5. It fails miserably to deal with mutliple same names under different parents
6. The turning off/pausing of the feature is very poorly designed
I love love love LR, but this feature is rubbish, and it could be such a useful and important feature for many of us. I am very dissapointed as I excitedly upgraded to take advantage of this feature, and it's completely unusable. The fundamental idea and design is ok, but the execution is terrible.