I replaced the spark plugs in a Toyota truck once, it was pretty easy, and a good way to learn different torque levels of my arm. Good experience overall.
I replaced the spark plugs in a Subaru once, it was one of the most ridiculously complicated self-repairs I've ever engaged upon. It involved dismantling nearly the entire engine compartment, and then utilizing those twisty/spinny socket extension things, as well as a system of magnets, and truly hoping for the best since visual confirmation of pretty much everything involved was completely impossible. I felt like a man afterward, but never want to do that ever again. This actually might explain why I bought a Mazda this time.