If you're looking for a serious big mountain AT boot, this could be your ticket. It's not the best for long tours, and it's not the greatest resort boot, but it is good for hucking 30 footers in the backcountry all day long. I would recommend these to anyone looking for a boot to use for big backcountry lines, with emphasis on the descent.
I was absolutely blown away at how well these boots performed for me. This is the only AT boot I feel comfortable driving a big ski with, and since I got these I have never looked back. I've never gotten blisters or even thought about my feet while touring, and on the way down they do really well.
However, don't expect alpine boot performance out of the Endorphins; they are still designed for going uphill, and for that reason a few things are comprimised. In order to really drive the ski like I want to I have to tighten them down a bit more, and even then ski control isn't quite 100%--maybe around 90-95%. Definitely top of the line for backcountry gear, but this is all relative.
There's good and bad things about the design. I'll start with the good:
The lace-up liner is really nice for tours of any length because it allows you to keep your boots loose and not worry about blisters.
Interchangeble soles are awesome if you end up using alpine bindings for a day, and the vibram ones are really nice for walking on rocks.
The velcro strap on the top has worked really well for me so far, it's noticably better than those found on a lot of alpine boots I've owned.
There's really only one thing I don't like a lot about the design of the boots, and that's the buckles. I've found them to be kind of awkward to buckle down since they're mounted the opposite way most buckles are put on. Personally, I haven't seen any advantage to this (in fact, it seems like they get iced up more this way), but it's really just a trivial issue I have with the boot.
If you're looking for a nice pair of AT boots make sure you do it with your wallet wide open. They don't tend to get marked down nearly as much as alpine boots do, and as a result you'll end up paying close to full retail (~$700) more often than not. But as far as value is concerned, I haven't regretted the decision once. If you're looking for a boot like this they're all in the same price range, so if it fits its worth it as far as I'm concerned.
As you can expect, these are super comfortable. From day one I was able to be in them all day without sore feet, and my toes have never gone numb (they almost always do in alpine boots). I even picked them over my snow boots quite a few times to work in last year, for reference. But keep in mind these are ski boots and fit is highly subjective, so definitely try on a pair before you buy.
As far as AT boots go, it doesn't get any stiffer than this. Although these are rated at 120, they are noticably softer than my Lange FR120's--maybe they are around 100-110 or so. This estimate goes out the window when you're as forgetful as I am and accidently ski in walk mode. Anyways, it's a nice even flex until you REALLY lean on them hard with a stiff ski, then there is a point where they will almost stop flexing. Overall, I'm really pleased with them as an AT boot: not super wimpy like most, but not uncomfortable to tour in all day.
Between working and skiing last year, I easily wore these 40+ days. If you don't look closely they could still pass for new. The liners are still absolutely perfect, but the shell and vibram soles got scuffed up from the rocks at Moonlight Basin--in defense, these are the sharpest rocks I have seen at any mountain I've been too, bar none. The buckles and walking mechanism have been flawless during the entire time I've owned them, so I have nothing to complain about.