The safety of this cipher is low and its interest is only historic. Its cryptanalysis can be made through digraphs frequency analysis. Since it is a polyalphabetic cipher, though, the Playfair makes the decryption a little difficult. Since there are more digraphs than letters, the number of elements available for analysis diminishes. It has other advantages: one doesn't need tables or complicated devices to crack the code - only a key-word, that can be easily memorized or traded. The key-word is easily implemented and little prone to error, so it is a perfect system to be used as "field cipher".