I've had a set of their NYXL .009's on my work guitar for about 2 months. My initial impressions were that they felt more typical of their cheaper XLs, nothing too different from a string tension or textural feel. I very quickly forgot there was anything "different" about them at all, to the point I haven't really thought about it until their intonation started getting weird about a week ago.
Sound wise they were about like their cheaper XLs as well. I believe my take was that perhaps D'Addario went more for an angle of trying to make a string that was basically undetectable as being different from their normal XLs, where as with the Ernie Balls they did have an atypical tension and surface sensation.
Like the Ernie Ball Paradigm "long life" strings, when they start to go bad they do it differently than "normal" strings. When the Ernie Balls go bad it seems like it's almost amazingly fast. I come to work one day (teaching guitar if the reader doesn't already know...) and suddenly they won't stay in tune, won't intonate, and are not pitch stable.
The D'Addarios seems to have started going bad a week or so ago, but in a much more normal, slower fashion. However, in a weird way - it's almost like maybe they're "bad" at different lengths of the string, or something has gone wrong at a certain point that makes certain notes more out than others? A very strange behavior - part of the string has lost it's properties but not another, perhaps? Deceiving, because you think it's in the ballpark with the octave of one note, but half of the notes are out.
They seem to have kept their sound the whole time. Very tricky - the intonation starting to go slightly bad combined with the sound staying more or less "fairly fresh" means I think I've been fighting them for over a week, spot-retuning constantly. Yes, now that I think about it - I have spent a lot of time tuning over the past week or 2. I almost forgot to mention, a did break the high E maybe 2 weeks ago.
CONCLUSIONS
The Ernie Balls last a bit longer. They also seem to be more pitch stable than "normal" strings, which is a bonus as far as I'm concerned. On the other hand, they feel a little bit more taut (which makes me think "are they really something odd like a half-size heavier?). When they go bad, they really go bad, out of the blue.The D'Addarios are more "normal" in all respects, but at first glance seem to not last as long. That normalcy in feel/tension might be a bigger plus for someone over the Paradigms.
The Ernie Balls are $15 on Amazon, seem to last a fair bit longer, and seem to be more pitch stable (more so than normal) versus the D'Addarios at $17 on Amazon. The pitch stability would make me choose the Paradigms on that alone, but they're also cheaper and last longer. I think Ernie Ball wins this round, unless D'Addario can drop the price to maybe $10-12? I know they don't want to hear that.....
POST SCRIPT
The thought just occurred to me that long ago I tried the Optima gold strings, who advertised a similar spiel: longer life, but not with the specific claims these brands do. My take on them was that they did last a little bit longer - just a little bit, and they did seem to intonate better but were also noticeably stiffer, particularly on the wound strings. Much more than the Ernie Balls. They were only about 25% more expensive IIRC, but I didn't think they lasted that much longer and the stiffness was bothersome.
The reason I bring this up is that now I'm wondering if they didn't use a different specific alloy, or a different quality control, perhaps similar to what Ernie Ball is using? I believe Optima was just pushing the gold coating as being more corrosion resistant and durable, but not the metal alloy itself? The metallurgy of the Ernie Balls seems very remarkable, I was very surprised by them, but perhaps the D'Addarios lasting longer-than-normal combined with a more typical feel is just as remarkable?
In both cases I'm profoundly surprised by just how much longer both strings seem to last/sound good/intonate relative to their cheaper versions. I was expecting something of an experience more like what I had with the Optimas long ago. I have NYXLs on another guitar at home, and will put a second set on my work guitar today, but unless one of those surprise me with their durability I'll probably end up being an Ernie Ball user after staying with D'Addario for a very long time.