Anandtech, Driver Heaven, HardOCP, HotHardware, Legit Reviews, Overclocker Cafe', Overclockers Club, PC Perspective, techPowerUP! and TechReport have published a review on the ATI Radeon HD 3870 X2. Here's an excerpt:

The design of the new AMD Radeon HD 3870 X2 graphics card, previously called the R680, is really quite simple at its core: take two RV670 GPUs, put them on the same PCB with their own 512MB frame buffers, connect them with a PCI Express bridge chip and enable CrossFire for a better performing single card solution. In reality of course we all have much bigger questions than that and AMD has provided most of the information we were looking for. R680 is indeed a good display of engineering that ties together nearly all aspects of its design into a neat package.  The target segment for the Radeon HD 3870 X2 is the highest AMD has targeted in some time: performance leadership. That's right, AMD is planning on taking the 8800 Ultra and GTX for a ride all while pricing the R680 at $449 - less than current 8800 Ultra pricing and about the same as most 8800 GTX cards.

Obviously, AMD has captured the title of "fastest single graphics card" with the Radeon HD 3870 X2. There is much to like here. The X2 is generally faster than the GeForce 8800 Ultra, and it has HD video decode acceleration that Nvidia's older G80 GPU lacks. In all, the X2 looks to be a pretty good value in a single-card, high-end solution at $449. The X2 does draw more power and generate more noise under load than the GeForce 8800 Ultra, but it's not unacceptable on either front for a card in this class. And the X2's seamless multi-monitor support is the icing on the cake. I'm really pleased to see that working so well.

That said, the X2's title is by no means undisputed. Nvidia's best alternatives to the X2 are based on the newer G92 GPU and have support for H.264 decode acceleration themselves. For just a little more money, a pair of GeForce 8800 GT cards in SLI will outperform the X2, usually by a healthy margin. Those 8800 GTs will come with many of the same multi-GPU caveats as the X2, however, plus additional ones about requiring two PCIe x16 slots and an Nvidia chipset. If you're hoping to sidestep those worries, one can hardly afford to ignore the solid value presented by the GeForce 8800 GTS 512 at around $300. The GTS 512's performance isn't far from the X2's in many cases. Unless you really are planning on driving a four-megapixel display, a card like the GTS 512 will probably feed your appetite for eye candy quite well in the vast majority of today's games