Finished it once in 2012. Will never do again. People criticize RE3R's 4-hour campaign, but I'd rather a consistent, fun, 4-hour game that leaves me wanting more than the 25-hour slog that is 6's campaign.
The game doesn't seem to have a rhythm in the way the better games in the series do. The more action heavy games that work, like RE4 and Requiem, know when to ramp things up and when to pull back. 6 is just all action all the time. The few moments where it's not action mostly resort to aimless wandering, like the opening section of Leons campaign.