What is your opinion of this product?
First off, the build quality is excellent. If you're purchasing this as a live mixer with the FireWire audio as an afterthought, the fit & finish is nice and you'll probably be happy but there are cheaper standalone mixers out there.
I bought this as a FireWire audio interface and didn't understand that the mixer controls, with the exception of the EQ, only affect the main outs, so you can't for example fade or pan what is going into your DAW. Strike one, but I could live with that. What ruined it is the software. Initially on receiving the unit I didn't know that it wasn't currently Mac compatible, because I was running an older machine with Mac OSX 10.8 the latest OSX supported by the drivers. But since upgrading my Mac, I discovered 10.9/10.10 aren't supported. You can enable support by disabling signed kernel extensions but it crashes constantly in Live / Logic. Wasted $650, too late to return now. Google Mackie Onyx Yosemite and see what I mean. Mackie support doesn't care.