i play practice matches against a guy 1/2 level above me and 15 years younger..i win about 1 out of 3-4 sets..His returns really bother me and i sometimes revert to an underhand serve..i can hit it well with sidespin. i win about 60% of the underarm serve points. Even when he senses it coming it takes him off the court (mostly in the ad court) and i still many times can win the point..To me it's a legitimate play.. I seldom try it in league/tournament play but i will if the occasion arises