I'd have to disagree on the statement that it was a "dangerous" gamble on AMD's part. I think AMD more or less saw how effective the value adds and features worked out for nvidia and wanted to emulate that somewhat, thus we have Mantle being AMD's value add. If Mantle is great, or good, or even half way decent. It will help sell their cards, period. Now we'll have debates forever about how good or bad Physx or TXAA are, but the bottom line is that there's a niche of users out there that buy nvidia cards because they enjoy the nvidia specific features. There are definitely a lot of people who buy nvidia because they say...."well I want g-sync next year. I like physx. Etc. Regardless of how someone else might NOT value it, it helps nvidia sell cards. So now AMD has that AMD specific feature or "killer app" perhaps that will help AMD drive their own sales. So they're basically doing the same thing here iMO.
We don't know how Mantle will perform yet. Like I said, it could be great or it could be just semi decent. But it will help AMD sell their GPUs regardless. I think it was the smart play on AMD's part. We don't even know how good or decent Mantle is yet and here we are talking about it 119 pages later. It created a buzz, that's for sure.
I really hope they continue down that path because I see some of the aftermarket 290X reviews and their performance is good and where it needs to be. (and without the reference issues) So they're good on that front. Software wise, they're not as good as nvidia. Yet. I would throw my personal opinion out there that nvidia is just hands down superior in terms of adding polished software features on a regular basis (for instance, TXAA, adaptive vsync, driver forced AO, shadow play, etc) So they're emulating nvidia in terms of creating value adds, hopefully they can push a little harder and get the software side up to snuff. That's the next move AMD needs to make. More software features - the last feature I can remember AMD adding was MLAA years and years ago. They need to pick up the pace on that kinda stuff. Then we all win through competition, regardless of which brand one prefers. So I guess, Mantle is the start of where AMD is headed. Hopefully they stay on the right track.