Before we move onto how the HD 3870 X2 performs, we should really talk a bit more about the merits of this whole multi GPU idea because although multi-GPU solutions have been around for a long time they've never really managed to convince completely, at least perhaps up until now.
We've seen a number of dual GPU graphics cards come and go over the years but they've seldom stuck around for long, been readily available, or used particularly elegant designs. However, it's none of these issues that have been the real sticking point of dual (or multi) GPU solutions. No, the real killer is the variable nature of their performance.
The problem is that for a multi GPU solution to work properly you need software - both drivers and games - to support it. If that support isn't there, at best performance will only equal what you'd get using a single card and at worst it could actually make performance worse than a single card or even cause the game to not work at all.
Now, these issues can and generally are all fixed eventually but it can take weeks or even months for new drivers or game patches to be released and, if you've just spent a few hundred pounds on a new graphics card, having to wait even a week is not going to sit well.
Of course, ATI has promised that supporting the X2 will be of utmost priority with ever more co-development work being put in to ensure games work properly straight out of the box. Moreover, it has also been suggested that multiple GPU is here to stay and we will be seeing single card-multi GPU solutions more and more often as both nVidia and ATI look for more ways to push the performance envelope. So, hopefully multi-GPU compatibility issues will be a thing of the past.
All we know today though is that performance will vary wildly depending on what games (or other programs, for that matter) you're running. During our testing we encountered problems with both Call Of Duty 4 and Enemy Territory: Quake Wars and, although we eventually got figures that looked correct, it took a fair amount of toing and froing with ATI and tinkering with our test system to get to that stage and even then the performance was mediocre at best. More to the point though, we had no such problems with any of the single cards.
Ultimately, whether you're willing to take the risk depends on your gaming habits. If you're the type that wants the latest games the day they're released then there's a very real chance, the HD 3870 X2 may not deliver the performance you expect immediately. On the other hand, if you're a bit more relaxed about your gaming and are willing to wait a while for new drivers or game patches, if they're needed, then the HD 3870 X2 will suit you fine.