Just replaced the plugs on my '04. Replaced the first four plugs within 50 mins, but when I was bolting the front passenger coil pack back on, the bolt head sheared right off. The bolt was going in fine and the head was about 1/8" from being seated on the coil pack; but then it started squeaking, and within four cautious quarter-turns with a socket wrench, it snapped right off, flush.
My brother and my dad (both experienced pipefitters) couldn't extract the bolt, so they drilled out the bolt and tapped a new hole for a slightly larger bolt. I'm really glad that I chose to drive out to my parents to do this, and that my brother was out there too, because I'm not skilled in this area at all. The new bolt bolted in just fine. I'm kinda paranoid about it, but realistically there's almost no force pushing the coil up and out, so all should be good.
The whole process took five hours, but other than the broken bolt, all went well. Just typical "why the hell do they stuff all these hoses and brackets and wires in here so tightly" frustration. The plugs were 141,000km / 7 years old, and one plug was significantly worn down more than the other five; hopefully this will solve my hard starts in the cold mornings.