1855 Half Dollars come only one way...With Arrows...and they are very popular as type coins, primarily because the With Arrows design was so short-lived. The purpose of the arrowheads was to indicate a reduction of the weight of the coins under the Coinage Act of 1853. The arrowheads made it very easy to visually separate the old coins from the new. The With Arrows coins effectively drove all the old silver coins out of circulation.
1855 Half Dollars are not rare overall, but they are semi-scarce in Mint State, where the typical grades are MS62 to MS64. Gems are very rare, especially in MS66, and anything better is exceedingly rare -- it they exist at all. Fully struck examples are almost impossible to find, as virtually every one we've seen has weakness (to some degree) on the stars on the right side of the obverse.