Have tried this multiple times over the years.
After a couple weeks, it doesn't do much for a larger melasma/sun damage light brown patch on my forehead, but it does appear to slightly start lightening a smaller (half thumbnail size) sun spot below my eye. It burned on application only after a couple weeks of twice daily application. I also noticed the dark spots where I put it immediately flush red after applying. May have also caused some tiny clogged pores where I applied it.
The problem is, it starts turning the spot red and crusting up like a scab at about week 2. And yes, I wear SPF 15 or 30 or 50 (depending on the activity) every single day. On medical sites, it says crusting is a sign you should discontinue use. One night laying off and using Olay Age Defying Night Cream (niacinimide based moisturizer, as the previous poster also mentioned), the scab peels off and the spot does look a little lighter.
Also, I read online that in those with sensitive skin/thin skin, this can worsen broken capillaries. On my forehead, I have 3 tiny new red blood vessel spots.
Probably works great if you're not sensitive to it and prone to broken blood vessels (which I am). I did see some lightening of the smaller, darker sun spot, but it's not worth risking it with the crusting, irritation, and new blood vessels showing.
EDIT 9/11/14: like the glutton for punishment I am, I dug this out and tried it one more time. Tonight, after getting out of the shower (where I used my Clarisonic and CeraVe foaming to wash my face), I applied this on my forehead, under eyes, and nose. Within minutes, my skin was bright red where I applied it and was starting to itch. I'm sure the clarisonic enhanced penetration and elf to an accelerated irritation effect, and I had to wash it off immediately. So now this is definitely going in the trash, rather than the cosmetic graveyard cabinet.