So I read the most negative reviews, and with a lot of them it seems to be a matter of misunderstanding.
1. When you begin to use an AHA or BHA exfoliant, it is normal for your skin to "purge" - that is, you may have increased breakouts because the exfoliation is drawing out the gunk from your pores. These are breakouts that would have happened eventually anyway. These breakouts should last no more than a few weeks.
2. Yes, it burns. Especially at first. This is also normal, and the burning should subside after a minute or two.
3. Yes, you're supposed to leave this on your skin - after cleansing (and toner if you use one) and before anything else.
4. The smell of this and other PC products is simply the smell of the ingredients, because PC doesn't use fragrances in their products. THIS IS A GOOD THING. Smelling weird for a few minutes (those smells fade) is far better than irritating the skin with fragrances.
5. Please don't put this or anything else on your face without doing your homework. Not only to avoid bad products, but also to understand what to expect from good products.
All that said, I don't find this to be my favorite AHA, or even my favorite PC AHA.
For PC I recommend the 5% Resist product.
For a non-PC AHA, I recommend Olay's Night Resurfacing Elixir.