I can see Jirachi ex being a neat tech, but only if you have enough Mew ex LM in the deck to copy the attack. I don't find myself running many of them. I find myself running less and less the longer I play. Mew ex is weak to psychic, a popular type right now, and the card just flops when your opponent drops a cessation crystal (or gardevior ex d) on the field. Because there is no energy acceleration in Destiny, you have to place every energy wisely, and when your opponent can drop a single card and ruin your past 2-3 turns of energy drop, I just don't like it. Sure, the metagame in my area is focused heavily on cessation crystal, but overall, I feel Mew ex to have low HP, a bad weakness, and a overall huge weakness to cessation crystal. I'd rather put my energy on Mew*, and only lose 1 prize and be able to abuse CC myself, or on Rayquaza ex d, and be limited but not killed by CC.
Just my opinion though.