I'm 22% through and I have to say its quite a (satisfying) challenge a lot of the time. I think my problem is I suffer from a mutation of Pokemon syndrome....Gotta kill them all!
It takes me ages to complete a mission because I find it physically and mentally impossible to ignore a big group of zombies. It's like Shaun of the Dead's Morris dancing scene, except its me surrounded by 20 zombies and I usually end up dead. Not before I've taken half of those mongs with me though! Of course playing it that way. Means I go through hard earned modded weapons like nobody's business, and usually end up having to fight the big b*gger at the mission objective with a floorboard.
That's just the way I roll.

It frustrates poor old Timey to death, I'm like the kid that lags 100 paces behind and he has to keep waiting for me lmao!