The difference in flavor between Pepsi and Pepsi Throwback is fairly subtle. The latter is a bit smoother and slightly sweeter. The Mountain Dew Throwback, on the other hand, tastes significantly bettert. The sugar-sweetened version is much crisper and the flavor bolder than its HFCS counterpart.
The packaging says that it's for a limited time only, however they are priced only slightly higher than the regular versions. Costco was selling the 18 count cases for $4.99, while a 36 count case of regular Pepsi or Mt. Dew usually goes for around $9.