Sezane quality is nothing like H&M! I have a few beautiful blouses and coats from Sezane- all are lovely. I would say the quality is more similar to Baukjen or Hobbs or maybe Boden. Better than Hush and Other Stories and definitely better than H&M, Nobodys Child, and Zara.
I would say not worth the money at full price though and to wait for the archive!
Saying that though, I have bought things at full price if I've really loved them and because they are so nice I've worn them enough to justify the price- even if they weren't necessarily worth it for the quality.