I don't like the play all that much. It draws additional attention. Maybe not as much as splitting 10's, but it does draw additional attention and drawing as little attention to myself is my top priority. I basically treat the play as I do splitting 10's. I will do so at very high counts. (higher than the index for the play) and make sure I am leaving at the next shuffle.
Doubling A9 creates even more variance than splitting 10's. With splitting tens you can turn your most likely winning hand into a double win, a push (win one lose one) or a double loss. With doubling A9 the possibility of that push is much smaller. You most likely will turn your probably winning hand into a double win or double loss.
Increased variance isn't a reason not to make the play for me. As I said, my reason is longevity concerns, but increased variance should be a consideration for some of the players on this site that seem to be playing under-bankrolled.