$20 extra for an "essential complement"? That's just a slap in the face.
I see no reason to purchase an Omnimic over REW and a calibrated USB mic. That little move just sealed the deal.
We've been over this several times in the REW thread and I believe the general consensus of those more in the know on acoustical measurements related to home theaters is that little or no value should be placed on measuring the interaction of the surrounds (provided the basics have already been covered i.e. trims, xovers etc.).
I paid $28.87 USD through PayPal (And have not received it yet)