Siggi‘s a little pricey ($2.50 for 6 oz., compared with $0.50 for yogurt), but if you like such things it is worth experiencing. However, you should probably know a few things about it before you rush right out and buy some.First, Siggi’s differs from authentic skyr in ways that my friends (who have tried the real thing) insist are "subtle and not important." Siggi’s is not made with the same bacteria culture, and is made with ingredients, such as agave nectar, which have not been available in Iceland until recent times.
Second, Skyr is a fresh cheese, and is substantially thicker than yogurt, but can nonetheless be mixed in most of the same ways to give body or smooth texture to a dish. It is frequently used in Iceland with and without sweeteners as a topping on pastries or in cereal dishes, or mixed with fresh fruit as a treat.
Thirdly, the sugar content of Siggi’s Skyr is much lower than blended yogurt. In my opinion the plain variety, having less than 1/6 the sugar of blended yogurt, must be mixed or sweetened to taste—it is otherwise too tart. In contrast, I find the fruit-flavored varieties are enjoyable right out of the cup, and have as much as 1/3 the sugar compared to blended yogurt.
Fourth and finally, my own extremely limited experience is that the closer the date on the cup, the more sour the skyr will become, but this can be overcome with a half-packet of sugar or sufficiently sweet additional fruit. There are also a number of ways to cure "sour" Skyr, but most of them involve taking the sour Skyr as a starter to make more Skyr, a process which requires Rennet and unpastuerized or slow-pasteurized milk. If you have the means and the drive, you can search the tubes for "Thettir" and "Skyr" and find recipes to make your own.