Ron Guth:
Of the four mintmarked 1908 Dimes, the 1908-S has the third largest mintage. The PCGS Population Report confirms the third-place ranking, with a population between that of the 1908-O (the scarcest) and the 1908-D (the second scarcest). In Mint State, the 1908-S is readily available across a spectrum of grades from MS62 to MS65. MS66 examples become scarce, but not rare. MS67's are really rare and PCGS has certified none higher. One of the nicest examples is the PCGS MS67 that once resided in the Simpson Collection; this coin has white centers surrounded by gold and blue colors at the peripheries (see the illustration above).
The strike quality of 1908-S Dimes is usually very good to strong.