I got both a BlackWidow (regular) and the Rosewill RK-9000 on the same day (ordering snafu, but I don't mind having 2 decent kb's).
Even though both keyboards use the same switch, the key presses on the BW feel "thunkier", having a lower pitch in the sound they make. The Rosewill feels "clackier".
The BW is 3.0lbs, while the Rosewill is 2.8lbs.
I hate the font on the BW keys. I also don't like that the F1-F4 keys are shifted a bit to the right. Seems unnecessary. I play Starcraft2 and the macro keys tend to get in the way of where my hand would like to rest, but who knows maybe these macro keys will prove to be handy whenever Diablo3 comes out.
The software that comes with the BW is decent, though for the life of me I can't get profiles to automatically switch when logging into SC2.
Other than that, both keyboards are awesome, though I'd say the BW is better value. Time will tell if build quality is in issue, but there's nothing obvious to indicate the BW is cheaply made. I'm still not sure which keyboard I'll stick with for SC2.