Iridium plugs are unlikely to have any affect on mileage or performance. They reportedly will last 90-100k miles, longer than platinum plugs (60k miles) and a lot longer than copper plugs (30k miles). That is their chief, maybe only, advantage. For some drivers, that is enough to justify the additional cost by not having to change the plugs quite so often.
Personally, I would only use them if I was putting on big miles every year.
The OEM Denso Iridium plugs that were in my Honda-powered Saturn Vue V6 were at 115,000 miles and still performing great when I changed them last year. They're what I'll be going with when I change them in the VDC.
I got them on sale at Advance Auto, and the price wasn't much worse than the average plugs I priced at the time. They're $8.99 each right now, which isn't really that expensive.
I have used NGK iridium plugs on all my cars every time plugs were to be replaced. They last a long time but I am not sure about any increase in performance or mpg improvement. I haven't measured that although there are some claims about the improvements.
I have a scangauge on my H4 automatic. The mileage has improved with the installation of NGK Iridium IX plugs! I was getting ~25 MPG on the winterized gas here in the NW on the highway @ ~65 MPH. With the new plugs (and nothing else changed including oil) I was getting ~27 MPG with much smoother power and more torque through the rev range. Now that I did change the oil the mileage probably will tick up a little more.
The 'old' NGK V-power had only 12k miles on it. I decided to swap them out since the V groove was over 1/2 gone with some uneven 'wear' (the groove on the center electrode was mostly gone nearest to the bend in the arm!). The color of the ceramics around the center electrodes were a nice light tan. Nothing out of the ordinary but they were tossed after driving the new plugs.
My experience with iridium plugs is that they do improve cold starting a bit, and cold running, as well, when "Cold" is below 20F. I believe this to be a product of lower voltage requirement, resulting in a better spark at very low temperatures. Besides that, they do not deteriorate for a very long change period.
Before I started using them in my Beemer motorcycle, it took a fair amount of cranking, and some increased throttle above and beyond the fast idle ramp. Now, I can actually start it without the fast idle, adding the fast idle after it lights off.....At least to the limit of temperatures I am willing to ride it in. (20F is pretty much my lower limit).
I called NGK and told them exactly what I was doing with the car (top end rebuild, and looking for fuel economy improvements where I can). They recommended: NGK BKR5EP-11 (3440) laser platinum. I even called back a second time to verify that.
We bought our h-6 with 132k on the clock. I now use it for a work beater and it has 206k on it..I have never changed them..must be the previous owner did though but I don't know..she bought it new but I never asked about the plugs.
on a 17 mile one way (mixed driving)trip to work i'm getting 21-22 on my scan gauge so i'm happy.