People here have something against flat serves and don't recommend them, but the pro's use them a lot no? Like last year's masters cup, Federer vs Nadal, Federer was down 0-30, and made 4 straight aces, and I'm pretty sure he made 2 or 3 flat aces.
I know that they do not recommend them much for those who don't have the skill, or technique, but if you actually practiced and spent time perfecting them, they'll become a deadly weapon.
I myself am 5'6, and I can serve flat serves, with a somewhat good %. I always use them for 1st serve, and if they don't go in, I have my back-up topspin serves, which are actually pretty good.
The only serve I have not yet learned is the slice lol.
Even with my height, and with pretty bad flat serve technique (because I don't get the chance to practice much, so it's not that good), I can still reel in many flat serves in matches, and aces too.