Hi there!
I got C4 as soon as it was available but am very dissapointed and still using SX3 for daily work. C4 runs stable and is all in all resonable bug-free, plugIn drag and drop and the new fx are nice BUT
- I need side chaining and my side-chaining fx are DX - a format which is not supported in C4 anymore, as somebody else pointed out.
- the one monster bug that kill it for me is: sample streaming is broken. It works in realtime but when you freeze a track or mix the song down in offline mode, samples that are streamed off the harddisc by your sampler (be it Kontakt1+2, DFH or Steinbergs very own Halion) get chopped off in the middle.
Personally I think if you got some decent third party plugIns and software instruments there's absolutely zero reason to upgrade to C4. Unless you desperately need the control room feature - I don`t I set different monitor-mixes for the musicians up via fx-sends on the channels and am done with it.
Just my 2cents, YMMV...