According to the instructions, you should apply on to perfectly cleansed skin and after 3 minutes wipe any excess off with a tissue (i.e don't rinse). I personally like to use this after my Dermalogica Gentle Cream Exfoliant so it penetrates into the skin better, or if my skin is feeling particularly dehydrated then I would leave it on overnight. By the time I take it off (even if it is just 3 minutes) my skin feels soft and toned- I can't comment on its firming or anti aging abilities as those aren't my problem areas yet but a lot of people swear by it.
There's no getting away from the fact that it is Sisley and so it is expensive, I think it is currently £72.50 in the UK or about 100 euros in Spain. Also, you do get quite a small amount of product- just 60ml so just over half what most brands would give you. Having said all that, is it worth it? Well, in my personal opinion, I love it! It is very hard to find a hydrating mask for oily/combination skins that isn't greasy or one that doesn't cause the redness in sensitive skin to flare up and this one fits the bill perfectly. I would however always advise that you go along to a counter and pick up a sample before investing in any sort of skincare product.