At first glance, the ASUS EAH5870 certainly makes a good case for itself, boasting an 850mhz clock speed, 1GB of onboard RAM, DirectX 11 support and to top it all off, easy overclocking with dedicated utilities.
Thankfully, the EAH5870 mostly lives up to expectations, performing exceptionally well on nearly any modern game you can throw at it. The card even performs well when running extremely modded games such as Skyrim, with frames being consistently high even when the game is burdened with all manner of texture packs etc.
The card also runs very quietly, with fan noise barely perceptible (in a well ventilated rig with many active cooling fans).
It's not all good though, as there is a downside to this. The card's DirectX 11 performance is not great. Tessalation in particular really makes the card chug. For example, on Crysis 2 DirectX 11 mode Tessalation drops the FPS from an almost constant 60, down to 15-20.
Overall though, this is a great card, with good speeds, temperature and noise level.
If you want DirectX 11 however, you are better off going for a 2GB card, or running two EAH5870s in crossfire.