I was looking into the Steelseries Merc Stealth keyboard because of the gaming pad on the left side. However, I was looking into the "Anti-ghosting" feature because I knew their claim to "7 simultaneous keys" was just not possible considering the USB interface it uses. In my search I have found only one person who seems to have tested it to any measurable degree, and this is what he said:

"I've recently been a bit disappointed with my new Merc Stealth. It doesn't live up to the SteelSeries advertised spec of "anti-ghosting up to 7 simultaneous keystrokes". Testing it (with Notepad and some decent free keyboard testing software), I found the following:
- Gamepad ("Gaming Terrain") all 6 main cluster [QWEASD], any 4 adjacent on [123456] row, any 4 within [CVFRBGTP] group"

Does anyone know any more than this? For example, would it be possible to press 2 of the main cluster and 3 of the adjacent row? Do the 7-11 keys perform like the 1-6 row? Ect, ect. One reason I ask, is that my cheap $12 keyboard can press at least 5 keys in the QWERTY block without ghosting or getting blocked. With that in mind, the only true benefit to getting the Merc Stealth would be the key position in the gaming pad. Everything else would just be convenience. Does anyone know of anyone else doing tests of the Anti-ghosting on this keyboard? If so, I would very much appreciate a link if you have one.

For reference, the post I quoted was here (you can skip to paragraph 8 for the results):