Like anything else, they have their strengths and weaknesses. In this case, the only thing keeping the Naxos from getting a 10 from me is the lack of a higher lift setting on the heel and the minor play in the toe when you really lean into them going fast. Aside from that, I have absolute confidence in these and would recommend them to anyone looking to ride bigger lines in the backcountry.
Downhill: I have just as much confidence charging on these as I do with my P14's. Granted they ride high and have a little lateral flex due to the toe pivot, but when it comes time to drop in on a line my bindings are the last thing on my mind.
Uphill: Amazing. The double pivot is so smooth it feels just like you're walking; none of that weird feeling you would get on Fritschis or Silverettas. The only drawback I've found is that the heel could use a higher lift setting for the really steep climbs. There have been a number of times where I've either been uncomfortable going up something because my feet are tilted too far back, or I've had to cut a new trail because of the lack of lift on the heel.
As I've already mentioned, I think the pivot system Naxo uses is brilliant. The only weakness is that is causes a very slight bit of play in the toe. The toe height is adjustable to accomodate for alpine and touring boots, and the heel has a downhill lock, so I really can't ask for much more without being ridiculous.
If you look around or get in on a group buy, NX21's can be had for ~$300. In my mind, that is very good for what you get in return. It's not quite a "you're retarded if you don't buy this" deal, but if you're in the market for a burly set of AT bindings it's definitely on the cheaper end of the spectrum.
So far I have been very impressed with the retention of these, they are very solid feeling for an AT binding. After 35+ days of touring last year I have had only one release--a nice double ejection after landing too far forward off a drop to flat.
So far these have been flawless for me. I've done everything from exposed straightlines at Big Sky to 25 foot drops in the backcountry on them and haven't had any issues to date. However, Naxo had problems with pieces breaking on their first run of bindings, so beware if you're looking for a used set. As far as I know this issue has been resolved, and I certainly won't hesitate to buy another set of NX21's when the time comes to replace mine.