I chose the Wilson Steam 99 over the 99S model, simply because I generate enough topspin on my own, and wanted a more stable frame. Ive found it to be an excellent choice for me to generate plenty of pace on ground strokes, while being maneuverable enough for me to put away crisp volleys at the net. Its 16 x 18 string pattern is open enough to generate plenty of spin, while stable enough to keep my shots consistent. Its a bit heavier than some of the team frame models out there, so its able to provide plenty of pop on baseline ground strokes. Its designed for those of us that have quick swing speeds, and consistently provides great feel all around the court.
I typically play aggressive tennis in the 4.0 - 4.5 range, and heartily recommend this racquet for serious competitors, whether your specialty is singles, or doubles.