I've been using the VM-3100 Pro by itself for several years and have gotten to know it pretty well. I am a BIG fan of this board. It can do a million things, and it does everything well. It has a pretty large learning curve, and too few XLR inputs...but if you take the time to learn how to work it, it is one powerful little mixer.
I added the RPC-1 card a few months ago and integrated my VM-3100 Pro into my digital recording setup. I was amazed at how easy the setup was, and how well it coexists with my M-Audio Delta 66. Turns out the Delta and the RPC-1 both use the same M-Audio driver and control panel! I was happily stunned by this discovery, as my studio suddenly jumped from 6 tracks to about 20. Sweet.
The one really big limitation of the VM-3100 Pro is that it can NOT handle sampling rates greater than 44.1KHz. This can be a real deal-breaker for a lot of people. I have issues with it, myself...but...I have learned to live with it for now. Luckily, it does handle 24-bit processing, which is its saving grace (or else it would be in the dumpster). I really don't mind working in 44.1KHz with 24-bit bandwidth. If I want to work with higher sample rates, I can always fall back on the Delta and use the VM-3100 Pro as an analog mixer. It all works out.
The effects on the VM-3100 Pro are very high quality, and there are hundreds of them. In fact, I have used the mixer as a stand-alone effects rack for my sax on live gigs. It really rocks for this and I have seriously considered getting a second unit just for this purpose. I have also used it as a sub-mixer for live recording, as a keyboard rack, input/output monitor mixer, MIDI control surface, vocal effects module, etc. It also has two onboard compressors which work great (very smooth and transparent), and both can be used in conjunction with the two independent FX processors.
I do wish it had motorized faders for studio work, but since the faders can be set to remember their positions digitally I don't find this to be a big deal. I do really appreciate the MIDI implemenation of this unit, as I can use the MIDI channel faders to control practically anything in Sonar by setting the remote control options. It can get a bit confusing remembering where the physical faders are in relation to the "virtual" faders when you switch back and forth from various audio and MIDI channels. The little transport is great, once you figure out how to make it work (you have to set up MIDI key bindings in Sonar).
The VM-3100 Pro really is baffling complicated at times. It's not too hard to accomplish something once you know how to do it, but I wouldn't call it "intuitive." For all its shortcomings, however, I still love this incredibly versatile little powerhouse of a mixer.
Let me know if you have any specific questions about the VM-3100 Pro, I may be able to help.