It's not very effective for ME for softsynth control. In plugin mode it can support a number of effects, but it's a pretty small list. I have an Edirol PCR300 that I use for that stuff. The ACT stuff works well with it and it keeps me concentrating on the seperate functions. The stuff on the keyboard is generally for keyboard stuff. I use the Behringer for actual mixing, mostly due to the motorized faders - takeover of the controls works ok in ACT for synth parameters and stuff, but it's just wierd when you try to use it after having switch the banks of the channels that you are controlling.
If you are in Mackie mode on the BCF, unfortunately, you can't just switch mid-stream to ACT mode. You would have to reboot the BCF to switch its mode and Sonar doesn't like when you do that when it's running, so exiting Sonar and going back in is usually necessary to get things right. This is, of course, terrible for workflow.