John Frieda's Go Blonder Lightening Spray really delivers on its promise. I'm a natural blond and quickly saw a difference after the first three applications.
My only concern is that it made my already dry hair (a problem many natural blonds tend to have) very dry and brittle. The bottle warns not to use it on your hair more than 10 times - I would have liked to use it more for a greater lightening effect but chose not to due to my hair drying out too much.
So if you're looking for a quick fix, I'd definitely recommend it, though in moderation. Add some heat or sunshine after application to boost effects, but wash it out after a day or two and be sure so condition your hair to the max!

For more lasting results that may take a little longer to achieve but are definitely less damaging to your hair, try John Frieda Sheer Blonde Highlight Activating Shampoo - after stopping the lightening spray I switched over to the Highlight Activating Shampoo and continued to see results, but enjoyed better hair condition.