I'm definitely not an expert when it comes to Isabel Marant, but I do really like the brand. I have a few pieces of her clothing and 2 pairs of footwear. I bought two new jackets last week on vacation in the US. What I really love are her prints, I find them quite unique, definitely statement pieces. It is kind on an "it" label at the moment, whether it is worth the price is kind of up to the individual.
Isabel Marant Etoile (the less expensive line) pricing I would say is on par with other contemporary brands. The mainline is really getting up there though. The quality is better in the mainline, but it doesn't blow me away either. The best pieces usually sell out at full price.