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Pro Performance can be improved via Alienware Graphics Amplifier
The Alienware Graphics Amplifier is essentially an external GPU that can be used to handle graphics intensive games. It includes 350-watt PSU and 16 x PCI-E slots so most cards can fitted within it. To use it, boot the laptop with the amplifier attached.
Pro Lightweight and compact
The laptop is an inch thick and weighs 4.5 pounds.
Con Poor performance and amplifier costs extra
Alienware 13 comes with a GeForce GTX 860M GPU with 3DMark's Fire Strike Standard Graphics 1920x1080 benchmarks showing a score of 3910. As of March, 2015 the top GPU has a score of 9774 so this GPU while reasonable now, won't be a good gaming GPU for long.
Alienware Graphics Amplifier that can be used to improve the graphics performance costs additional $299 + the price of a desired GPU.
Additionally the i5-4210U CPU that comes with the laptop is scoring just 3,443 in PassMark's CPU benchmarks while the top option is just above 10,000. Since there are no alternative configurations, the CPU load will considerably slow down the overall performance of the laptop.
Not in production.
Con Poor display
The laptop has a 13.3" low resolution 1366 x 768 TN-Panel that's not especially bright at 200 nits.