I don't need "experience" of any sort to know that a GPU is broken and gimped out of the factory -- I've read plenty of complaints both here and on GPU vendor forums to know that it has serious throttling issues and needed to be BIOS flashed in order to even work properly or ALLOW the GPU to sustain a decent overclock and that was from people who spent $3000+ grand on their GPU setups and water cooling alone. And I have a friend that owned one and returned it for the exact same reason. Prior to doing my fair bit of research, I had plans to move up to the Titan from my GTX 660 just before reviews for it came out. I ended up disregarding the Titan and returning my GTX 660 for similar reasons (throttling/unstable clocks, and a flatout nonsensical TDP, which was exceeded in several games under normal gameplay load), except the stupendous $1000 price tag of the Titan, which I was ready to fork out because I even had the cash set aside, had it been as good as promised by Nvidia.
And Mantle has direct ties with AMD's consoles, all of which run on AMD GCN GPUs -- regardless of how effective it will be, it is of more value to the average user than all of Nvidia's bullshit gimmicky features combined, including PhysX and the rest of the proprietary garbage they run. Since Mantle promises to make porting to the PC easier, it has a far brighter future ahead of it than anything Nvidia has to date.
And BTW, the GPU market share on Steam includes a crapton of old GPU users like me, who are running Nvidia's golden age cards like Fermi and G92, who will at some point be looking to upgrade. Fact is AMD is taking more market share from Nvidia and faster than it ever has before and lots of people with money to spare, like me, will switch to AMD without a second thought if the performance is there. The only reason I am holding off pre-ordering a R290X right now is because A. I have a 3D Vision 2 monitor, which I wish AMD supported as well as Nvidia, but they don't, and B. My nearly 4 year old, VRAM starved GTX 460 still manages to run BF4 beta at a half decent framerate. The minute it dies or I find something tempting enough to upgrade to, I will.
Oh and BF4, just like BF3 and BFBC2 before it, will be the main, if not the only reason I will upgrade, so I don't need to "wait and see" to know how Mantle will turn out. BF4 will be the deciding factor in my purchase, so if Dice say "Mantle makes a 290X mop the floor with the Titan" or "gives it an advantage over equivalent or overpriced Nvidia cards", it is already of more worth to me than anything Nvidia can promise.