I have 94k on my '05 now and purchased with 18k so I'll chime in...
Overall, car has been reliable but has had issues.
-Rear axle click. One of the first annoyances I had. Minor, though. Repaired under a TSB.
-Passenger seat power motor switch failed.
-Driver's side seat track replaced (at least twice).
-Had original battery explode. I replaced it at night and saw the acid damage on my fender and wheel the following morning. That was an unfortunate and expensive fix.
-Compression rods replaced once. Installed rods were updated part, have been silent for many thousands of miles (over 40k).
-IPDM module replaced. This left me stranded. Very rare failure, most will never have it fail.
-Radio caused battery drain. Came to a lifeless car one day after leaving work.I know it was the radio cause the lights were off (always leave them on auto). Had radio replaced many times and eventually replaced with aftermarket.
- Camshaft sensor replaced. Almost left me stranded. Car would stall but would restart.
-A/C compressor became noisy but stil cooled 100%. Replacement cured noise.
-Couple airbag failures. Had passenger seat airbag replaced two weeks ago. Can't remember the other component.
Can't help on the spark plug/valve cover issue but I will remove them this weekend to inspect since my warranty expires next week.
Fortunately, most items above were covered under my extended warranty. Best investment I made with this car and paid for itself at least twice.