It has never clogged my pores or caused breakouts. On another board, it worked for about 90% of women who tried it; the other 10% seemed to have no results at all, so that does happen. At least I've never heard of someone who's had a bad reaction to it!
For me, it only took a couple of days to notice a softening of my skin. I think the other results, such as fading of red marks and rapid regeneration of skin, takes a bit longer. Of course, it varies from person to person.