If they have feelings for you and you don't have feeling for them, then No. You're playing with their feelings and that's not cool. Now, if neither one of you have feelings for each other...it's simply casual and cool, then I say sure. At one point in time you were something like friends, no reason why two adults can't hang out. (read: drinks or casual dinner,
not BFF joined at the hip-there is a REASON they're your ex.)
I let an ex take me out for drinks once, but I must admit, it was moreso because he had seriously 'let himself go', and I on the other hand, had maintained and then some,lol...so it was one of those "Eat your heart out" dates, lol.

