I used to run strictly AC plugs. After reading Toms's posts about NGK plugs I gave them a try because I was having a miss at cruise rpm in my new 406. After putting the NGK plugs in it, I've had great results. The engine runs so much better. I have since put NGK plugs in everything else I own and I noticed a performance/drivabilility improvement in each vehicle. Whats great about it is that they are only $2. I have an auto restorer magazine at home that I recently got in the mail and they did a comparison on 12 or so different spark plugs. They ranged in price from about $1.50 to $12.00. The regular NGK plugs won the contest.
The next time I go home I'll try to scan the article in so everyone can read it.