The underhand serve is an effective weapon. Use it as part of my arsenal. I am able to generate some very nasty spin, the ball stays low and takes a wicked bounce. It is difficult to return. I would estimate that I win 8/10 points when I call upon this serve in match play. My teammates love it when I use it on others. Some of them that have seen it enough can handle it in practice. I don't use it much for fear it will lose effectiveness. Don't ever let my opponents get too many looks at it. Maybe once a set, usually to the ad court where it is most effective for me.
I also hit a topspin forehand serve on occasion. I am right-handed and stand at the far outside doubles line of the deuce court in doubles. I take the ball out of the air and using a forehand motion am able to generate excessive top plus a mean angle to boot. This serve is harder to execute, but the angle I get, when hit correctly, puts the opponent into the fence (or wide) to make a return and the whole court is then open. Still perfecting this little gem. Get more angle than I can serving over the top.
Serving is like pitching. Some folks are blessed with great fastballs that have some motion. These folks can just walk up to the service line and fire their good stuff at you and be effective. Others, like myself, have average fastballs and need to rely on location and change of pace. Sort of like the Greg Maddux of serving.