Hey Studio X. Just curious what OS and model Mac you are on. I also loved my Mackie Onyx but can't get it working with anything past Snow Leopard. It turns out there are some Onyx boards that cannot be updated with drivers/firmware and thus become big paperweights once you're on a new computer. I'd keep my old machine but really want to use newer software such as Logic X.
Anyway, there's more info at the link below from Mackie. Based on their info, I'm even considering buying a used 1220i with one of the later serial numbers to give it a try.
srosehill - you may want to check into this before buying an Onyx. I see that you're on Mavericks and there's a good chance you'll end up with great peice of hardware that's unsuable with your mac.
http://www.mackie.com/products/onyxiseries/drivers/
JD