Super interesting thread, since I very recently transitioned from a moderately high-energy dome Sierra-1 NrT to a RAAL Sierra-2. I also had a pair of regular Sierra-1s without the NrT tweeters in my office, so I've recently been through three different tweeters in the same cabinets. Now, the S2s have a different woofer to handle the higher crossover, so the RAAL isn't the only difference between the various iterations, but it's a big one.
I would rate the base S1s a little laid back, the S2s neutral, and the S1 NrTs as a little forward. In no case did I think the difference was dramatic; all of them are still pretty neutral. The S1s are not as laid back as, say, many Wharfedales, and the NrTs are not as forward as, say, Klipschs.
I don't think the difference between the NrTs and the S2s is necessarily dramatic, depending on what you listen to. If your primary material is "wall of sound" compressed rock or popcorn movies where explosions are king, I'd be maybe be a little wary about the price difference between the RAALs and the NrTs. (Note this is not a knock on rock or explosion movies—I love both of these things and they are mainstays for me as well. But there are other things in my listening universe.)
What the RAALs (or maybe the package of RAALs and the S2s Curv woofer) did deliver relative to the NrTs is:
* Wider soundstage with more "air" between instruments, if available in the recording. Not a huge difference for some music, but a marked difference for classical.
* Better retrieval of detail.
* Faster transients.
Fundamentally most of these things don't help all that much when what you're listening to is someone wailing away at the E-string on the bass (I can say that, I play bass—or rather, I'm learning to—and what bassist doesn't enjoy a good hammering of that big old E?) or blowing up a helicopter. You know, I love the Clash like they were my brothers and while they sound better on the S2s, they don't sound that much better on the S2s. If that were my target material, the upgrade probably wouldn't have been worth it.
But I've not heard anything remotely in this price class that makes a recording of a violin in a quartet sound more like being in the same room with someone playing a violin in a quartet. Full stop—that's a big deal. For me, worth it right there. (A lot of Pink Floyd is also much better on the S2s.) But if that's not your thing, then sure, I can see thinking that the RAAL hype is oversold.
They're not miracles, but I do think the S2s are better than the S1 NrTs. But to me, how much better depends on what material you're throwing at them. And maybe your room, too, as there's someone on the Ascend forums who went back to NrTs after upgrading to S2s because he didn't like how they interacted with his room.
And it's not like what is achieved by the RAALs is necessarily impossible to achieve in a dome. I've heard good and bad domes, and I'm sure there are good and bad implementation of ribbons. In fact, the first time I heard ribbons 15 or so years ago, I was totally underwhelmed by what I'm pretty sure was a terrible implementation. The upper mids/lower treble were scooped out like the engineers had just crossed them over as if they were standard domes. And there are all kinds of new, exotic domes that a supposed to be amazing—even DaveF has fiddled with some of these. What Toole said is almost certainly right that it's not an intrinsic property of the technology being used—it's not especially that they're RAAL ribbons that's important. Instead, I'd say it's that they are good tweeters with certain design properties that when utilized well deliver good results.
Study: Schiit Bifrost Multibit > Yamaha A-S500 > Ascend Acoustics Sierra-2/Emotiva S8
Living Room HT: Oppo BDP-83 > Denon X3400H > PSB Image 4T/8C/S50/Rythmik L22
Office: Schiit Modi 3 > NAD C740 > Ascend Acoustics Sierra-1 NrT
Bedroom: Onkyo TX-8050 > PSB Image 1B/SubSonic5