While the HD600 is universally praised, and it seems to be 50/50 on whether someone prefers the 600 or the 650 (the HD600 is the flatter, more accurate headphone, the HD650 is "HiFi") opinions seem to be very mixed on the HD700/800.
The HD700 was universally panned on release until the price was reduced. (I believe it launched at $1000 and is now $750 MSRP)
Opinion these days seems to be that it's a good purchase, though there are a lot of options in its price bracket.
The HD800 seems to be a love-it or hate-it headphone for many people. It's very revealing, and needs to be paired up with a suitable amplifier to sound its best.
There are also issues with ringing at higher frequencies,
which people are resorting to DIY modding to fix.
It's somewhat frustrating because I was not really in a position where I could justify spending the money on an HD800 back when they were first released six or so years ago.
And now that I can, I am concerned that there's a good chance they will be replaced with an updated model that fixes this if I do decide to buy a pair now. I have a habit of making big purchases right as companies decide to introduce new models.
Of course that may or may not be an issue for you. There are plenty of people that enjoy the unmodded HD800 sound, and if you really enjoy it, you probably won't care if a newer model gets released.
There's been a lot of competition in the high-end headphone space recently from companies like Audeze and Oppo, and there have been murmurs about a new flagship from Sennheiser over the last year or so.
There were similar murmurings about a new flagship from Sony about six months prior to the MDR-Z7 launch (though I don't recommend the Z7) - and Sennheiser have been bringing the Orpheus to trade shows again recently.
So it looks like there could be a new model released some time this year, but it may be a flagship along the lines of the Orpheus ($13,000) rather than an HD800 update/replacement. ($1,500)
It's not my intention to cast doubt over this, I'm just voicing my own concerns as someone that has been thinking about the HD800 recently as well.
But if you're anything like me, you'd rather be aware of it than find out after making your purchase.
And as you will be very aware, who knows what the future holds? You could be hit by a bus next week - so why wait 6+ months just because something better
might come along?
While I have generally been a "wait and see" type of person when considering large purchases like that, there's been a lot of things that have happened over the last 6 months or so which are making me reconsider that.
I'm now leaning more towards: if you want it and you can afford it, just buy it. And if you can afford the difference between the HD700 and HD800 ($750) I'd just go for the HD800.
A few years down the line I'd always be questioning whether I should have just spent the extra $750.
Somewhat related, you may find Linus Tech Tips' recent Sennheiser factory tour interesting:
https://www.youtube.com/watch?v=5es8zggYM7AAnd while I know that a lot of people don't like Ken Rockwell's opinion on things - especially now that he's moving into audio instead of just photography, I do find that his reviews are useful to get a more practical idea of what something is like, without a lot of the audiophile "baggage" getting in the way:
http://kenrockwell.com/audio/sennheiser/hd800.htm