My fox is a piece of cake to get at the plugs. So changing them slightly ahead of schedule doesn't bother me. The 2V and 4V plugs aren't that bad either so again I like to fiddle.
My hemi was an absolute nightmare. I originally called the dealer because I was just going to have them do it.
For low grade cheap copper plugs they told me over 600 including the parts(I forget the exact cost). I told them over my cold dead body will I pay that kind of money for a couple of hour job.
Well it took me 6 hours to change 16 plugs. The ones behind the brake reservoir were a total nightmare. There was no room to have my socket,extension,swivel and ratchet all assembled and slide in the hole. Nope. I had to assemble each piece,blind,so it was all by feel,and re-install was the same scenario.
There are 5 iirc that are a pain because of the brake booster so the ones closest to the firewall on the driver side,but only 2 on the passenger side were difficult.
When I was done I reflected on the effort,frustration and pain from getting poked everywhere because I had to lay on top of the engine and I re-considered the quoted price and felt it was worth every penny for me to not have to do it again,expense vs reward,and the expense was worth the reward of not having to do it.
I used platinum/iridium plugs so I could take advantage of the extra time in between plug changes.
I do like standard copper plugs in cars that are easy to change. Over time we don't notice the loss of instantaneous response,the speed of start up,ease of passing and those couple of mpg lost due to less than perfect plugs however once changed we all notice how much better the engine runs,so with cheap copper plugs I don't spend alot to maintain that level of responsiveness,and I really can't tell that they were changed.
I feel that I rather maintain top level of performance,rather then keep the platinums in or longer,and they gradually lose their performance,but because they do last longer I just accept the slug my motor becomes.