I'll start...
 
Comping and Melodyne were the huge ones for me. Next are QuadCurve EQ flyout, browser enhancements, and track colors; I don't use the tape emulator much but when I do, it's what the doctor ordered.
 
What I still need to check out are Strum Acoustic (ignored it initially because I play guitar, but the modeled sounds are quite cool), most of the Blue Tubes plug-ins (I have a lot of "old faithful" plug-ins and tend to favor the ProChannel anyway), Gobbler integration, and the BiFilter 2. Haven't done much with AD, but that's because I use mostly audio drum loops. The few times I played it with, I was impressed by the editability.
 
How about you?

So much new stuff made it to my PC since the X3 release, the biggest feature of X3 which I appreciate is stability with Melodyne a distant second. I have used comping a bit and the QuadCurve a little more because it is handy but all the other stuff; AAS, Blue Tube, XLN plug-ins, Gobbler, audio snap improvements... get no play. I really would like the QuadCurve HP/LP filter in a separate PC plug-in. This would get a lot more use than all the 3rd party plug-ins bundled in X3.

I use the most, by far , Pro-channel, I think I have all the available modules, and I will do insanely( by my normally entirely sane standards) complicated audio routing to able to use them on all my instruments. I think that I have opened the Loop Constructor ( or whatever it's called, I don't have Sonar at hand) View maybe once.

I am looking forward to using drum maps one day.

scook
I really would like the QuadCurve HP/LP filter in a separate PC plug-in.

Is this so you could put it in a position in the signal chain different from the where the QuadCurve resides?

Yes, that is the idea. Multiple QuadCurve EQs would also be nice but a simple HP/LP filter does not need much screen real estate.

worstcaseontario
I think that I have opened the Loop Constructor ( or whatever it's called, I don't have Sonar at hand) View maybe once.

Well the question was about X3, but...I use the loop construction window all the time. Not just to create and edit loops, but to do varispeed, fine-tuning of samples, and using "wrong" settings to create electro loops that parallel the unprocessed loops.
 
Hmmmm...seems like I should do an article on "Why the Loop Construction Window is Cool."
 
But back to X3...I think the feature I'm going to investigate next is Gobbler. It occurred to me I could use it as temp backup for works in progress.

The ARA implementation of Melodyne was the highlight of X3 features for me.

 

Which X3 features do you use the most, and which have you not explored yet?

 
You may find this hard to believe (I do), but I bought X3 at the beginning of April, and I just got it downloaded.  So, the answer would be "all of them", to the second part.  I read with great interest (as always).

If the Loop Constuctor does not qualify for this thread, then you seem to have named what I replace it with. I only looked into Gobbler to figure out if I had any actions to take to shut it off. It's harming me in no way though.