As the researcher behind the "anti-ghosting" feature of the X4, maybe I can give some insight. By the way, I am not speaking for Microsoft here - this is just my personal take on things.
You are correct when you say that the X6 suffers from roll-over issues. However, this is not unique to the X6. This is a problem that almost all keyboards have, including most gaming keyboards. It's fundamental to the basic matrix design. In a nutshell, this type of keyboard can typically report any 2 keys pressed together, but some fraction of 3 key combinations (typically less than 5%) do not report correctly. Many manufacturers rearrange the internal matrix to try to make sure that the bad combinations do not include the ones used in popular applications. (e.g. CTL-ALT-DEL has to work!) This is kind of equivalent to sweeping the dirt under the rug - you really didn't solve the problem, you only moved it someplace else where you hope it won't be noticed. Some manufacturers make a big deal out of their rearrangements, and talk about certain regions having "anti-ghosting" and they talk about those keys being able to be pressed simultaneously. What they don't talk about is pressing those keys with any others outside the region. It is not uncommon to find that pressing a single key outside the region causes problems inside. Of course, this depends on the particular design.
The X6 uses the matrix rearrangement trick. I guess you could debate if one keyboard has a better rearrangement than another. Certainly, some choices work better for some games than others.
I would rather have a keyboard that just didn't have this issue. They exist, but they tend to be very expensive. The X4 is an exception. This keyboard uses resistive multitouch techniques internally that allow it to recognize any combination of keys, up to an absurd number. Before the X4, the standard way to get this sort of functionality was to add a diode in series with each key - a rather expensive proposition. Although the X4 is a less expensive keyboard than the X6, its "anti-ghosting" feature competes directly with high-end gaming keyboards. In this respect, it is far superior to the earlier X6. Of course, the X6 has features that the X4 does not. But for me, knowing that "what I type is what I get" is a far more valuable feature than a relocatable numpad, or knobs. This is a matter of personal preference, and others might reasonably decide differently.
As a side note, the ability to press lots of keys simultaneously on a USB keyboard exposed a bug in earlier versions of Windows. When a large number of keys were pressed (>11), Windows would sometimes generate false keypresses. Some reviewers (including the one here at Tom's) incorrectly attributed this to the X4. This bug was patched in Win 7 SP1. This fixed the problem, not only for the X4, but also for other USB keyboards as well. If you do get an X4, I highly recommend getting the latest Service Pack.
Finally, here are some pointers to additional information:
Lots of information on ghosting and the specific capabilities of the X4:
How to change the backlight color on an X4 (will void your warranty):