Odds are, if you're looking at these boots, you're also looking at the Dynafit DyNA and maybe a few others- and let me assure you that if a) the boot fits, and b) you can afford it, overall, you've found the best of the lot. I have spent 1 season on the DyNA, 1 season on the Alien 1.0, and will now be racing on the PG 400/Dynafit DNA- but if you can only get one of the three of these boot pairs, this is the one. (I'll try to review the DNAs separately once I've spent more time on them).
Everything everyone says about it, all the raving, etc., is true. These really are amazing boots that are outstanding for races, but also as a general, "legit" ski mountaineering boot that you can take into the mountains (weather appropriate, given the lack of full coverage on the boot). The ski performance is astounding for what they are- the carbon makes the boot really stiff and capable of comfortably driving skis with widths up into the high 80s (mm). They are almost two stiff at times- in mank/wet powder/heavy snow, for example, the lack of "progressive flex" will be a little jarring and you will get kicked around, but you will be able to drive the skis as you wish. As has been extensively covered by Jonathan Shefftz and others, the boot is exceptionally durable, and other than the lever breaking occasionally (not a problem I have had) they can handle a fair amount of mountain abuse.
One more racing-centered thing I really like about these boots is how easy the transition is- just flip the lever and forget it. As you stand up the lever will find the notch itself, saving you precious seconds of fiddling around vs. other options like the DyNA and PG/DNA. It's as easy as it gets.
One downside is that the boot is a PITA to put on/take off, especially with the gaiters. But if you're leaving it on for the day it's not that big of a deal. Also, the boot does have significant exposure to the elements- for that reason I have never felt comfortable taking it into the deep mountains or in very cold weather.
One other small racing-related point- the DyNA has a rockered sole which makes it the best for running out of the lot, whereas the Alien 1.0 has a flat sole which makes it good for mountaineering but not as good for running (feels more like an ice boot and clunks around when moving fast). While comparing it to the DyNA, it is also a) significantly stiffer, with b) a more secure boot upper/lower connection, and c) the lower on the DyNA tends to bulge/buckle under pressure, reducing the performance even during lateral movements- this is not a problem on the Alien 1.0
Lastly, the boot is somewhat "high volume," particularly above the foot- so if you have a flatter foot, you may need to raise your foot with a insert for a good fit.
Overall, 5 stars for a costly but excellent boot based on a revolutionary design that should give you many seasons of use.
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