My "new-to-me" 2006 S'wing had 9100 miles when I got it in mid-May and earlier this week with 10,500 on it I felt I should change the spark plugs which were likely original. Having run an iridium plug for over 5 years in my 2003 Burgman 400¹ I sought out iridium alternatives for the stock nickel-chrome conventional plugs and found Autolite's XS4303 iridium plugs for $5.99 each at O'Reilly's. Not stocked locally but they had them the next morning.
The original plugs did not look bad:
The gaps were worn a bit, 0.038" and 0.037", but probably re-gapped they had another 5k to 7k in them if one wanted to be frugal.
So; with the Autolite iridiums idle and lower rpm running are noticeably smoother, and higher rpms are definitely smoother.
Of course a new set of conventional plugs would have likely have had the same effect--the proof will be "in the pudding" 10k miles from now.
BTW, a nifty tool to have is a torque-limiting plug socket:
They are available in various torque values and hex sizes, somewhat curiously I have found the 16 Nm (11.8 lb·ft) 5/8" socket to be nearly universal for 10 mm gasketed and 14 mm taper seat plugs--spot on for the S'wing, and close enough on the other vehicles we own (a 2003 Mustang Cobra, 1998 Mercedes SL500 and a 2003 Toyota HIghlander).
¹ - I ran an Autolite iridium plug for over 45k miles on my '03 Burgman 400--with no performance issues of any sort.