The 1858-S Half Eagle is more rare than the mintage suggests, with an estimated 100 survivors to satisfy collector demand. Akers wrote that he knew of no examples better than XF, but today we know of more than a dozen AU examples. Whether this is a result of grade-flation or the emergence of some nice examples is irrelevant. It simply means that collectors have more options than they did two or three decades ago. Unfortunately, no true Mint State examples are known. Yet...
David Akers (1975/88)
The San Francisco Mint began production in 1854 and, of course, the 1854-S is one of the great gold coin rarities. The dates from 1855 to 1857 are also rare, particularly in high grade. But beginning with 1858 and extending to 1876, the San Francisco Mint Half Eagles are, without exception, extremely rare better than EF and some are actually unknown (to me at least) above that grade. The 1858-S is the first of that run of S mint coins and it is a major rarity in any condition. I personally have never seen one better than EF and no more than a few even at that level. This date, and all the S Mint coins that follow until 1876, are very underrated and underpriced in all grades.