I've never had DD-X fail, even with open bottles a year old. I did have one crystallization event, but that I'm sure I can blame on a very cold winter chilling down the basement. Very happy with the results, always look wonderful.
I also keep HC-110 around, for work which would be breathtakingly expensive with DD-X, like running the Nikor sheet film tank, or developing a roll of Verichrome Pan 122 in 28 ounces of developer in a Nikor reel and tank. Also, the fog-inhibiting properties of HC-110 are good for processing 20, 30, 40, or even 50 year old Verichrome Pan.