Planars are a different technology, they have some good pros but a few cons too. Sound-wise, planars usually are faster and give better texture to the sound, revealing some details that would be really hard to reveal with dynamic drivers (unless you spend a shitload of money on some Focal or something like). I have Fostex T-x0 and Hifiman HE4xx, the Fostex require some mods to sound good because stock they have a rather bad resonance-like sound, and the Hifiman are amazing, period.
A friend of mine prefers the warm and relaxed sound of the HD6XX, and I also love them too, specially for low quality recordings or poorly mastered songs (a lot of new songs are awfully mastered and sound like shit on very detailed headphones, just listen some vocal trap and you'll know what I'm talking about).
The HD6XX have just enough detail to be revealing enough, but at the same time they forgive some minor flaws and make everything sound "just right". HE4XX are way more detailed, and they can make some well mastered songs sound GORGEOUS, but at the same time low quality music sounds like shit (typical trash in - trash out).
Tonal balance is also quite different: HD6XX have a very balanced sound matching almost perfectly the Harman target frequency response curve, witht a touch less treble for less fatigue and a bit more mid-bass for a warm sound, and they lack some sub-bass; HE4XX have better bass extension, with more sub-bass and treble, but they're not exactly balanced, I'd say they are a bit bright and mids are pretty forward, they can cause some fatigue and a slight sibilance on some tracks.
If you never had open back headphones before and wanna know what's the difference, I strongly suggest you to buy the Superlux HD668B: they're bright, detailed, have a touch of bass, and with better pads (without those thick foams in between your ears and the drivers) they're a keeper you won't regret having. They're super cheap and sound pretty good, you can't go wrong with these, and this way you can wait for the HE4XX drop with something good on your ears ;)