I think its pretty great the pricepoint Sennheiser has taken the HD600's to these days, and now Massdrop has even taken that price down further.
I bought my first pair of HD600's about a decade ago for $400 after wanting them for almost a decade at that point. I remember the first time I ever auditioned them was back in about '97 or '98 when they first came out @ J&R. I remember them costing an UNHEARD OF (pardon the pun) at-the-time $650, but just that first listen had me coming back to try them again and again for several months, telling myself I would eventually own a pair. They just knocked your socks off with their beautiful sound.
Once I finally owned my own HD600's, I basically stopped listening to music with speakers. I would look at my rig and if not for home theater use, the whole thing seemed a waste. Because now, all I ever did was jack into my XA7ES's headphone jack, turn the volume to 7 or 8 and space out. These headphones were my first venture into reference cans. I can not overstate how much of a difference it made to my music collection. All of the sudden I had to re-listen to CD's I had listened to hundreds of times and hear them again for the first time. These cans changed the way I looked at music.
Then last year, when the HD6XX's came and I got to own a HD650 lineage set of cans. The differences are there, and I can't honestly say I really know which I like better yet. I'm having a hard time ruling out nostalgia/bias for a set of headphones that I will NEVER let go of (I've replaced some parts over the years on them, because things do wear out (cushions, cords, headband) but its totally worth it.
If you don't own a HD600-series, be it, HD600, HD650, HD660, or HD6XX... do yourself THE BIGGEST FAVOR, and get an HD6XX when you have a chance. You will enjoy your music anew.
Added bit of advice, if you want to listen to quality coming out of your mobile device, try the Creative Labs E5 and play FLACs to it. It's as good as the mobile world gets.