I thought I'd post a comparison swatch between my three favorite MAC taupes for those of you who are considering buying the Sweet Sienna Pigment.
Now, Sweet Sienna is a limited edition item and unless you live near a CCO and manage to get a good deal on this beautiful pigment, the only other option you will have is to order from ebay.
The pigment costs about 30 USD on ebay. Quite pricey, but if you are a taupe lover like myself, you will most probably be willing to pay that amount to get your hands on this beautiful pigment.
I swatched Sweet Sienna next to two other taupe eyeshadows that I have.
(Swatched on the inner side of my underarm, probably an NC20 - primed with TFSI)

As you can tell Sweet Sienna seems to be the coolest toned out of these three. In the pot it appeared to be a mix of the two shades. But applied on my arm it was even cooler than Smoke & Diamonds. It also has more shimmer to it than the other two shadows and seemed to be a lot deeper and more pigmented.
Smoke & Diamonds is another Limited Edition eyeshadow from the Love that Look & last year's Starflash collection. It is a cool toned taupe, however it does have a slight golden sheen to it. This Starflash eyeshadow applies like butter.
Satin Taupe is the warmest out of the bunch with a strong plummy undertone. It is a permanent eyeshadow and a must-have shadow in anybody's collection (IMHO :) ).
So what's the verdict?
Overall I think Sweet Sienna is a very unique pigment and definetely worth getting if you are majorly into taupes.
Hope this helped.
♥