I read somewhere in the forum about Sonarworks and the headphone callibration tool they offer (as well as their separate monitor callibration version).
 
I just downloaded the 21 day free trial.  Seems worthy of evaluating.  Even though I will and do intend to mix most of the time with monitors, I'm thinking it can't hurt to have my headphones give a better impression of the sound.  If this plugin works as intended, seems like it would be a good value at the price.
 
I'll note my thoughts after I've played with it for a period of time.
 
There have been previous threads about it in the forum.  Here's one from around April:
http://forum.cakewalk.com...added-m3200874-p2.aspx
 

If you decide to purchase use one of these codes for 20% off.
CakeW-20
GLENNHP

Carl that's great to know. Thanks a bunch for that!

I used the trial and found it to be really useful. I got a much better, accurate mix using my headphones. I am due to buy the full product, so massive thanks for the discount codes. 

So, first off, the 20 percent discount will be nice.  That puts it at $55.
 
The download was smooth, install slick and easy.  The interface looks nice and has several options, but these are intuitive and it's very easy to figure out how to make it work.
 
I put it on my Reference bus, which is a bus I have after my Master bus.  I have Magic AB on this bus.  I put it after Magic AB.  When doing a mixdown output I turn off the Reference FX.  But prior to a mixdown output, I can listen to what's passing through this Reference bus (as an output from the Master bus) before it gets to my audio interface.
 
Turning on Sonarworks, and picking a couple options I was shooting for, I could tell an immediate difference.  It was almost disconcerting, but instinctively correct (like the difference between when I've mixed something down and taken it out to a different speaker scenario and the sound wasn't the same as what I heard in the studio).  There's a Wet/Dry control as well, so if you don't want the whole truth, you can scale it back.
 
The kicker for me was when I turned on Magic AB (which precedes Sonarworks in my Reference bus).  When I did an AB compare between one of my songs and a well mixed song (the one I chose was Throwing It All Away from Genesis). 
 
Without anything in my Reference bus on (either Magic AB or Sonarworks) my song as it's mixed presently sounds pretty sweet.  Then I turn on Magic AB and compare my song to the Genesis song.  Obviously the Genesis one sounds pro and better, but they both sound pretty good side by side.
 
Then, I turn on Sonarworks.  My song sounds muddy (still listening through Magic AB).  The Genesis song still sounds pretty great.  In other words, when not using Sonarworks, my song next to the Genesis song in Magic AB sounded decently in the same ballpark for mixing clarity.  When I turn Sonarworks on, my song downgrades considerably in mix quality, and the Genesis song still sounds right.
 
So, assuming I have Sonarworks on when I'm actually doing next steps in mixing, I may be able to hear and detect some of the changes that I may need to put in place to bring back the sparkle.
 
This may be a very short trial.  I like it!

It's well worth the money to me...it certainly makes a difference...