The 1921 is the second lowest mintage in the Walking Liberty half dollar series, second only to the 1921-D, and is very rare in circulated grades...definitely one of the key dates of the series. In mint state condition, the 1921 is also very rare. There are probably 40 to 60 Gem MS65 or better coins that survive. So in Gem condition, while certainly not as rare as the 1919-D or 1921-S or several of the other early date Walkers, the 1921 is a very rare coin that can be difficult to locate.
Like all Philadelphia Mint Walkers, strike is not a problem with the 1921. The typical choice mint state specimen, when located, typically shows very well struck detail of Ms.Liberty's skirt lines, hand, and head, and the eagle's breast feathers. Luster and color can range from a satiny pure white to deeply toned and everything in-between.