Using what I said and the car, If you've had the same car for a few years and have been listening to hundereds of albums and at different volume levels, you have learned what it sounds like now. So then you pop in your own creation and and should be able to compare what you hear. 
 
Like I said, I don't care how much you fuss and spend. it's all about learning to listen to any given system. Of course the better the system the easier this is to achieve. 

bitflipper
Yet it remains a useful reference because it will often exaggerate weaknesses in your mix.

actually, 
i've heard amazing pro mixes sound like sh!t in a car just as well!
 
honestly, i don't think listening to car stereos helps your case at all.
it's all over the map.
 
 

I have finally achieved a pretty good result comparing my studio monitor, headphone, and car stereo response to my mixes. Yes there are noticable differences between all three but they are now acceptable. 
It took me a long time to get my speakers adjusted by width and depth compared to where I sit. I then use both AKG240's AND ATH M50's to check and then my car. 
Another big deal for me was where I placed my mastering limiter. Now I use ToneBooster's Barricade set at the same bit depth as my song (usually 24) and place it Post Fader. This was recommended by ToneBoosters and at least now I can do a rough mix, upload to my Android and listen in cans and in my car and it all stays together.
 

Regarding Redline Monitor, FWIW, I'm ditching it.
 
On the one hand, I find it makes it a touch easier to listen while mixing, but I've decided I don't want to get used to that...

I use headphones a lot, but also my monitors.  I compare my mix to a reference song in both systems.  My spare room studio doesn't have any treatments yet though, so that's a problem I have to deal with.
 
Lastly, I usually put my final mix/master on the iPhone along with the reference song and play them in the car that has the better sound system.

An interesting article in SOS:-
 
http://www.soundonsound.com/sos/jan07/articles/mixingheadphones.htm
 
If only I could afford £1400 for a DAC headphone amp 

One area I think that headphones excell at is the small details. When I mix only on monitors I sometimes miss the small things I can pick up on headphones...like little clicks or pops..the sound of my arm rubbing the guitar after a take..the click of a mouse. 

I just bought a pair of Sennheiser  HD 650 headphones about a month ago. This is the first pair of cans I've had that sound really close to my monitors, especially the bass response. Some mastering engineers use them. They are a little darker than the Hd 600 phones, which mastering  guys seem to like a lot as well. I use them on every mix now for bass and spatial stuff to see how much my room is affecting what I hear and to make those type of mix decisions. I have a set of AKG 240 's that were my go to cans, and while quite good, the Hd 650 is far superior, at least to me.

I think the VRM box is incredibly useful. First of all, if you're completely limited to headphones then it's going to improve your mix's translatability immensely. But even if you're not, the box gives you a number of different listening environments to check your mix against. This is 100% more convenient than burning mixes to CD's and taking them into your car, over to your friend's house etc. It doesn't not matter whether or not the simulations are particularly "realistic" version of the listening environments and speakers they claim to be. What matters is that when you listen to a good mix through the VRM box, it translates well to all of the simulations. By that I mean the instruments are well balanced against each other whether you set the VRM to KRK's, Adams, Auratones, laptop speakers, TV speakers or 90's Hi-fi (to list some of the VRM presets). Nothing sticks out too much, the bass part is audible on small tinny speakers, instrument separation is good etc. I've listened to a lot of classic mixes through the VRM box, and they work through every preset. A tinny TV speaker isn't going to give you a great listening experience but a good mix will project the essentials of the music through it in a balance that communicates the soul and essence of the track. A great song that's mixed well will sound great even when played on a cheap AM radio. 
 
I think where a lot of people go wrong with the VRM box is in expecting to be able to do all of their mixing through it. Wrong! I believe the correct way to use the VRM box is to mix through whatever speakers you have (whether that be monitors in a treated room, monitors with ARC or a good pair of headphones) and to use the box frequently to CHECK YOUR MIX DECISIONS  through a few of the simulations to get an idea of any problem areas that need tweaking. For example you might find that your bass and kick parts are completely inaudible through the laptop speaker preset, or that the vocal jumps out too far on the Auratones. It's a balancing act of course. You're finding EQ and compression settings that work in many listening situations. 
 
It seems to me that the more listening environments you have at hand to check your mix through, the better. 
 
Any discussion about mixing through headphones is incomplete without the usual warning about keeping levels low. This is especially true in the case of earbuds, which can cause a lot more damage than over the ear cans due to the proximity of the sound source to your eardrum. It's important to keep the levels lower than you would ordinarily listen to, because you're going to be wearing those cans for a lot longer on a mixing project than you would if you were just listening in a recreational context. 5-6 hour sessions are not uncommon, and that's an awfully long time to wear headphones. If I'm working with headphones I stop for a break every 15-20 minutes. Tear 'em off and do something else for 5 minutes. And make sure you're getting enough magnesium, because a deficiency will leave your ears more vulnerable to noise damage.

@sharke... Zactly. The onus really is on the engineer to use the VRM box for what it is. It's a handy tool but there is a lot of technique involved too and imagining yourself in those virtual rooms. I have not been doing a lot of mixing lately but one thing I've tried to do is drag a reference track of a song I am very used to listening to into my project (which of course is always a helpful trick) and blast that through the VRM emus to get comfortable to the environments and then compare that with what I'm working on.
 
I've of course listened to a lot of my favorite albums a lot on all sorts of different sources but I've also been in a lot of cover/tribute bands. So there are certain songs/albums/bands I listened to over and over and over and over again dissecting every little nuance of the music. Those are generally the tracks I use especially if the material is similar. I know how those songs respond to various systems and what I should be hearing. That gives me a much better idea of what I'm doign wrong in my own mix.
 
I've been so preoccupied with tracking since I've gotten the bugger though I haven't had much chance to really do any massive mixing stuff with it. I do intend to revisit some older stuff and if I can ever get my current original project finished it will certainly be used (and already has a bit) for the final mixes.
 
Cheers.