My 2003 Explorer with 250k blew a plug and after doing quite a bit of research, decided to buy an insert kit. My other option was to have the engine pulled to remove the heads on it. The job went pretty good, but be sure to take your time and don't hurry it.
I actually had to do the job twice because the first insert failed to hold. I had used permatex red threadlocker and the damn thing came loose after about 30 miles. Luckily I heard the characteristic tick tick tick sound and knew what was going on. I ran the tap through a second time to make sure the threads were good and clean, then used brake cleaner to clean the grease out that I had used on the tap. Be sure to use grease or vaseline on the tap to keep the majority of the debris from falling into the cylinder. I vacuumed the cylinder out with small hose and blew it out with compressed air. I then flared the insert a little before installing it and used some of the max strength JB Weld on the insert threads this time. Also use some never seize compound on the insert installer tool that screws into the insert. That will make it easy to remove it. Let the JB Weld cure for the full 24 hours (I let it cure for 30 hours). Install the plug and go. Mine has been running great for 1000 miles since then. Still gets 20mpg on the hiway with 250k on it.

Some additional things I learned along the way are to make sure the piston is near the bottom of the power stroke. This way you know the valves are closed and the piston is clear out of the way. I accomplished this by using a piece of 1/2" garden hose with one of the spark plug boots taped to the end. I stuck it in the hole that had no plug and with all of the plug coils disconnected, I could sit in the car and hit the starter a little bit at a time until I could feel a little puff of air come out the hose end. You then know the piston is coming up on the compression stroke. Then I took a 24" long tie-wrap and stuck it down into the cylinder with one end sticking up where I could see it. I then hit the starter just a tiny bit and when I see the tie-wrap go down, I know it is on the power stroke.

I also have a cheap usb bore scope that I got from ebay for about $20. It came in real handy for checking the status of piston position and the results of my thread cutting, etc.

It's a good idea to have a good torque wrench when doing plugs. Ford specs plugs for my engine @ 25 ft/lbs. I set them at 26 with no problem.

In retrospect, the most important thinsg to me were to take lots of time, back the tap out every turn or two and clean the cut threads off and regrease. It is a slow process, but will keep you from tearing up the threads. Also when you are done cutting threads, use plenty of brake cleaner to clean those threads. JB Weld needs to have a clean surface to adhere to. I used a flexible grabber tool and put some rolled up paper towel on the end to get down into the cylinder and soak up as much of the brake cleaner as I could (didn't want a lot of that getting past the rings into the oil). And lastly let the JB Weld cure the full time and use the max strength long cure kind, not the quick set stuff.

Hope this helps anyone fighting this terrible design problem that Ford has blessed us with. Sure beats having to pull an engine.