well I LOVE the mackie onyx preamp, I use the onyx-820i desk and can't recommend it enough, but even though it has firewire, with my particular setup I have still found the M-Audio 192 card to have much lower latency, so I'm using it purely as an analog desk into the M-audio card and it works flawlesly, mic preamps are a highly personal choice I think.
I really like the 'colouration' of the onyx sound, it's very subtle, but some people hate it