This was the name of Blake Carrington's long-suffering second wife, played by Linda Evans, on the 80's prime time soap "Dynasty". Using it today will plant you firmly in the era of big shoulder pads and bigger hair.
"Krystle" sounds lovely when spoken aloud, but the spelling is rather messed up. Having grown up with a first name constantly misspelled, it can get really annoying very fast. If you're considering this name, keep in mind that the name will be misspelled all the time. Plain ol' Crystal may be a better choice.