The 1856-S Quarter Dollar is semi-scarce in low, circulated grades, but quickly graduates into a condition-rarity at the About Uncirculated level. Mint State examples are extremely scarce, with just a handful of examples certified between MS61 and MS65. The finest example is a single PCGS MS65 which seems to have disappeared into a collection, as it has not yet appeared at auction.
Be sure to look at the mintmark on the reverse of any 1856-S Quarter Dollar. One of the more interesting varieties shows a small S tucked beneath a much larger S. This variety commands a hefty premium over the "normal" versions.