This is my HG (holy grail) concealer. I have actually recently started using it as a foundation as well and it outperforms all of my other foundations in terms of wear.
I am about N10 and use Light, although it is slightly dark for me, and I may try it in Extra Light next time I repurchase.
I do not have under-eye discoloration so I cannot comment on how well it hides that, however it works great on my acne scars and general redness.
To use as a foundation I just dot (several dots on each area) on my cheeks, chin, forehead and nose and then blend with a foundation brush (like the Sephora 56). I find that it does not break down during the day or collect in fine lines, and the coverage (although full) looks as close to flawless as I can get with a foundation. I do still have to put a bit more concealer on some of my darker acne scars, however not much more & it lasts all day.
Even though using this as a foundation is expensive since it is $22 for about a third of an ounce I think it is worth it.
The only thing I dislike about this concealer is the packaging (it is packaged like a lipgloss) since I would prefer it to be in a tube for easier dispensing, however it will not stop me from repurchase since the formula is so great.