99 ML 320 wih 207K miles
I changed plugs over the weekend. Took about 30 minutes of prep (check gap, anti-seize threads, dielectric both ends of each plug wire) and about 90 minutes to do the R&R. Thanks for the excellent write ups in this forum. My experience:
1) I was really surprised how deep the plugs fit into the side of the block. It's like a Hemi. I wasn't expecting to be working mostly by feel. But didn't run into any problems.
2) I read in multiple places a 17 mm wrench is used to pop the wires off the plugs. I checked and a 17 mm wrench may have worked but it did not fit around the metal shaft of the wire. I used an 18 mm. It fit perfectly.
3) I did not remove a wheel or inner fender. I did remove all coil packs as I went and removed the air box to do the passenger side.
4) I wasted about 20 minutes chasing a new plug I dropped into the nether regions of the engine compartment. Got smart and stuffed an old bath towel in the gap between the engine and inner fender. The towel trick saved me twice after that.
5) I bought Beru wires (part number ZEF 1442 and 300.891.442 on the box) and I have a concern for the next plug change in 100K miles. Look at the image. The old plug wire is on top. The new Beru is on bottom. The Beru has much less metal and no lip to grab onto. The only thing you see when they're installed is the rubber part of the connector. Not sure how I'll get those baby's off in four years. Hopefully the dielectric will enable me (or future owner) to pop them off by hand. I may proactively pop them off and re-seat them every six months or so (if I can remember to).
Notice also my old plug with right at 100K miles on it. The center electrode is worn to a nub. The gap probably opened from 1.00 mm to 2.00 mm. Amazingly, the truck ran smoothly with the old plugs. Wife needs to give me some gas numbers from before and after so I can see if it made a difference in mileage. But this is still hard evidence you should change them AT LEAST every 100K miles.