The 1920-S Walking Liberty half dollar is relatively available in the lower circulated grades (G to VG), but is scarce in higher circulated grades. Like all early Walkers, this issue is rare in mint state as very few examples were saved at the time of issue. It is comparable in rarity in both mint state and Gem condition to the 1920-D, 1923-S, and 1928-S...rare coins all. While not as rare in choice condition as the 1921-S and 1919-D and a few others, the 1920-S is definitely a key date in the series in top condition. Many of the surviving Gem examples come from a 40 piece mini-hoard found by half dollar specialist James Bennett Pryor and sold to dealer Jim Halperin in the late 1970s.
The typical mint state 1920-S has soft frosty luster and/or some degree of golden toning. You would want to avoid the overly dark examples. As is the case with many S-Mints in this series, strike can be a problem, with weak detail of Ms. Liberty's left hand being common.