Older MBP`s with dGPU should be avoided, they are all literally time bombs, as validated by Apple`s recent warranty extension of the dGPU across multiple models (2011 - 2013). The Retina models have significantly improved cooling equally the dGPU is still pushed to the limits and in time will likely fail prematurely, as per Apple`s expectation.
Anyone thinking of purchasing one of the afflicted 15"MBP`s should think very carefully as the odds are against you. If you have owned the system from new, if the dGPU was used lightly, if you have been successful in moderating the systems temperature, if your lucky, or you can always join the 10`s of thousands of owners with dead systems. Doubtful ? one petition alone to Apple has more than 30K subscribers, let alone those who are not tech savvy, and not aware of the same issue.
The MBP with dGPU is a "buyer beware product"; if you need the dGPU for work purpose etc. then it makes sense as the cost of the machine will be rapidly recovered. If you don't need the dGPU avoid it, as there is a good chance it will fail after 2-3 years of use. If you want to game buy a console or build a cheap gaming PC, as gaming on the MacBook Pro is an expensive proposition.
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