In our initial launch reviews of the HD 7870, we praised how much performance improved over the last generation, and that still stands. The Sapphire HD 7870 Flex offers performance close to what NVIDIA's last generation GTX 580 flagship provided. Sapphire chose to not overclock their card out of the box, so the Flex runs at the same speed as the AMD reference design. I feel that they missed out on an opportunity here, as the card can easily run higher clocks. In our OC testing we see GPU clocks well above 1200 MHz. Memory reaches 1365 MHz, which is quite a bit lower than other HD 7870 cards, which reach 1600 MHz. This is due to Sapphire's choice to use Elpida memory chips instead of Hynix, which tend to overclock better.
A unique feature of Sapphire's Flex Edition is that it can support three DVI/HDMI monitors without the need for an active DisplayPort adapter. AMD has designed their HD 7000 Series cards to provide two TMDS clock signals. DVI, HDMI, passive DP Adapter consume one, while an active DP adapter needs no clock signal from the GPU. As a result, most EyeFinity users without DisplayPort monitors will have to buy an active DP to DVI adapter, which costs between $30 and $60. Sapphire's card doesn't need such an adapter, which will save you that cost.
Fan noise of the card is significantly better than AMD's reference design, especially idle is extremely quiet. Under load the card is quiet too, but given the low temperatures, it would have been nice to have a little bit less fan noise. Temperatures are great, much better than many other HD 7870 cards out there.
Sapphire is asking $310 for their card, which is not unreasonable given that the HD 7870 reference design retails at $290, and an active DP adapter will cost you $30. On the other hand, you can find a GTX 670 for $400 now, which supports three monitors, too, without adapter and provides about 30% more performance, which might be crucial to drive your three monitors at high framerates. But overall Sapphire did a great job with their card and managed to add a unique feature to their product that other companies do not have.
Edit, Jul 23: As Sapphire just told me, the second BIOS on the card is running 1050 MHz, so you do have some kind of option for overclock out of the box, it's just not enabled by default.