Iridium spark plugs

Just wondering how many miles I can expect from Iridium spark plugs? Are they worth the extra money?

 

I had them in the Bonneville but to be honest couldn't really tell if they made any difference.

 

I used iridiums in my old cb500 and it definitely started easier and ran smoother. Put them in my Sprint and no noticeable difference,in fact i read somewhere that they are not recommended for the carbed Sprints tho unsure why.

 

Bonnie had carbs so may be the reason it felt no different

 

I`ve got a 2001 Bonneville and these plugs did make a difference when new. However, I`ve now got about 11,000 miles on them and reckon they could do with changing...