I was putting off this task after reading the horror stories about #6. Now, after changing all of the plugs in 1/2 hour, I can say that the Xterra is FAR from the worst out there for changing plugs.

I'd like to add a few tips of my own to the list of helpful stuff already mentioned in this thread:

Make #6 easier my getting some things out of the way. Remove the vacuum line to the brake booster. Remove the bracket holding the throttle cables and move the cables. Even removing the plug wire from #4 helps.

Use the before mentioned 6" extension-universal-6" extension-plug socket setup to get the plug loose. Then get the universal and top extension out of there and use your fingers on the lower extension. Start the new plug using your fingers on the lower extension. If you can get 8 full turns on the new plug by hand then there is no way it's cross-threaded. Use the universal and top extension again to tighten the plug.

An inspection mirror definitely helps for #6. By all means use compressed air to get all of the crap out of the spark plug wells before removing the old plug. The odd-numbered side had tons of crap on the wells.

My plugs has only 51k miles on them and I'm glad I changed them. The gaps were at about 0.045" and both the center and ground electrodes were eroded. The replacements are NGK Iridium #BKR5EIX-11

Now for the old-timers out there (like me) I submit the worst vehicles I've experienced for changing plugs:

1) The late '70s Chevy Monza with the 262 V8. The engine has to be jacked up out of its mounts to get to #5 and #7.

2)1986 Chevy Celebrity with the 2.8L V6(my fathers). It took 2 hours to change the rear 3 plugs. Fortunately the intake manifold gasket failed and dumped coolant into the oil and fried the engine before it needed a 3rd set of plugs.