Well, I've had some experience with the 99 over the last couple hitting sessions because I've broken strings in the 99S.
I had the 99S strung with 16g 4G and I got about 3 1/2 hours out of it so I had to switch off to the 99. Today I hit for about an hour with Lux Alu BB (ran out of 4G) strung at 58 just like the 4G. The Alu BB lasted less than an hour.
So this is obviously going to be an issue for me with this racquet. I think 4 hours of heavy hitting is fair and I'm able to live with that but anything under that and I may have to reconsider using this racquet.
I know there was some discussion about a 15g 4G string but I can't remember if that is actually going to happen or not. I hope it does because I may put the 15g in the mains and put 16g in the crosses- don't know. Or just go with 15g all the way through. Anyone know for sure about the 15g 4G?
I still love the 99S and prefer it over the 99 only if I can get 4 hours on a string job.
The 99 is just what you would think it would be. It's a little less powerful, hits with less spin, and feels a little more solid due to the tighter string pattern. I think the 99 is an excellent racquet in it's own right. I had mine strung with Lux Savage at 55lbs and would probably prefer 4G 16L at about 52-54lbs in this racquet. Probably 52lbs.
I have some more 4G on the way so I will continue to play test the 99S and see how it works out. I have already made up my mind that one of these is my new racquet. Hoping it's the 99S.