Last summer i replaced about half a mile of 3 rail wood fence with 4x4 posts. I still have probably another mile to go or so, and i learned some valuable lessons. Number 1; it isn't so much building the fence that's hard, but taking away the old one. Logistically, that starts to pile up quick. 2) if you want it really straight, you'll be doing some 'cleanup' on nearly every hole with manual diggers. (i also did not buy a huge auger, theory being it'd limit the tamping) 3) no, it doesn't, you'll be tamping and tamping to get the dirt in the hole. Tamping is annoying. 4) in places where the previous fence was set in concrete, it was totally rotten. 5) Painting it is really a pain. (i ended up getting a sprayer). 6) old fence board nails show up in the strangest of places, like your foot if you're not on eggshells 100% of the time.
I even pre-sealed my treated 4x4s with paint in hopes to help long term.
Nearly 6 months or so after i sat my last post, i look at where i started and where i finished and can tell that I started to improve. I'm over all pretty pleased, but i wish i had started with the other end, instead of right smack dab in front.
I just bought a fence post driver to help me finish the remaining mile or more (depends on how i end up partitioning this area off).