With the 3870 X2 AMD has managed to design a card that is finally able to compete with the performance of NVIDIA's high end offerings. The card is based on CrossFire technology which makes both of its GPUs work together for more performance. One major drawback of CF is that scaling is very dependant on the application you are looking at. We saw amazing performance in 3DMark03 for example, where the 3870 X2 could gain over 60% compared to the 3870. In high-resolution Splinter Cell 3 this is even 100%. But then there are benchmarks like Crysis and Supreme Commander where the card performs only as well as a HD 3870 single GPU card.
Since some benchmarks show the massive potential of the HD 3870 X2 we can only hope that over time time there will be improvements to the driver that help with performance. Compared to two HD3870 cards in CF mode the 3870 X2 is easier to use because you just plug it in and it works, there is no need to change any configuration or activate anything. Also it will work on any motherboard that has PCI-Express support.
The use of two GPUs increased the power draw and heat output of the card considerably resulting in a noisier fan as well. What I found more annoying than the fan noise was the constant changing of the fan speed. If the fan is just a bit louder it will blend in with the other noises after a while, but if it constantly changes its speed your attention is drawn towards it. This can easily be fixed by using smooth steps between fan speed step which is possible on the RV670 ASIC.
The lack of PCI-Express 2.0 support is only very minor in my opinion. It has been shown that there is very little performance to be gained from doubling the PCI-Express x16 bandwidth alone. Overall the HD 3870 X2 shows good potential but the CrossFire implementation in the driver needs to be improved a bit before we can expect massive sales of the card.