So far my experience with the Mackie Onyx pres has been fantastic. For my rig I'm basing everything off of the 1200F at the heart of it all. I absolutely cannot wait for the 1200F to be released.
For what you are getting, I think it's a great deal. You could get the RME FireFace800 for the same price (or $100 less I think), but you don't get as many features IMO. The 4 seperate headphone outputs, along with seperate mixes for each one, is a fantastic feature...from the reviews so far the headphone outputs are powerful as well. Meaning you won't have to get a seperate headphone amp, just plug the cables straight into the phone outs and you're good to go once making everyone happy with their mix. Dual ADAT I/O, dual MIDI I/O, 12 Onyx preamps w/ 48v...I love it.
For the first time in a while I'm really excited to build my rig up again, and I gotta say it's because of the 1200F and the antipation I have for it.
But in the end though, it's up to you and what you are looking for. I know the 1200F will definitely cover my needs for a great while until I need to expand...at which point I'll just be adding an 8-channel ADAT preamp to it (DigiMax FS).
~e.a