I also use the X3 Live. My understanding is that the Pro version is a rack unit rather than a floor pedal and has more extensive ins and outs (including phantom power and two mic preamps). Here is my list of Pros and Cons:
Pros:
Two independent effect chains
Great amp and preamp sims - huge variety
4 mic sims and room effect
Excellent effects
Flexible ins and outs (two 1/4 instrument inputs, mic input, MIDI in/out, XLR ins and outs, 1/4 outs, SPDF, USB)
Great Software (Gear Box, POD Farm)
Variax input lets me set up Variax Guitar and Stick in the same unit for performance.
USB Audio interface and ability to assign outputs let's me process Mainstage/Reason keys, guitar, and stick in the same unit.
Flexible pedal assignments
Does not have the latency problems of software based sims
Cons:
Only one amp at a time for each chain - can't put a preamp in front of the amp
Generally not as flexible as purely software based sims like Guitar Rig, Amp Designer, etc.
For an all-in-one box, I think it is one of the best. For my purposes (running keys off a MacBook, processing Stick and Guitar) and providing MIDI control, I am not aware of any other product that covers all the bases. Here is song I did using the X3 Live for the Stick (also for the acoustic guitar and guitar solo at the end):
http://www.reverbnation.com/tunepak/2016467 (One Second After - first song). Sound on Sound gave the X3 Pro a favorable review last month. Note that the audio interface is USB and not Firewire.
Jeremy