I'm kind of surprised this has gotten no replies yet. Though I love the house, ISM is one of my least favorite Lutens scents. For some reason it smells of lipstick on me, then gets a little sweeter, then the lipstick again. I recently tried a few iris scents against each other and this one came in last. I'm also not sure it's something I would wear in hot weather. There's something thick and heavy about it, even though it seems more streamlined than other Lutens.
Encens et Lavande and Gris Clair might be better Lutens options for summer wear.
For hot weather and iris, if you haven't already, I'd check out The Different Company's Bois d'Iris.