I also recently got to try out the Rooth lineup; my thoughts are actually quite similar to those of average_joe's (which is why I'm not creating a new thread), save for a few small details. I might do a detailed review one of these days, but for now, I've merely posted short thoughts of each model. Take these impressions with a grain of salt, as they're largely formed from unprocessed information reflexive of initial thoughts.
The overall Rooth sound is indeed tilted toward the brighter side of things, but has a smoothness that suits my listening tastes well. Rooth has brought out a product lineup that has more of a Japanese-style sound signature, and I can see why the brand is so popular in Japan. The sound is also consistent across products; anyone can hear that they're from the same family.
LS2: ...is actually quite neutral, but it is way too sensitive and just not refined enough in certain areas; it also has a slight bass boost. I quite like the sound, though. It's a good entry-level sound that has great potential as a neutral-ish monitor.
LS3: Er... I'm not a fan. I took it out after a few minutes. I should give it more of a chance as I'm sure it's not as bad as I think, but I'm feeling a little lazy.
LS4: ...Not bad for a bass-driven BA CIEM. I like the way the highs are presented (present, but not overbearing), and it has nice mids, but is placed just a bit too far away for my personal tastes. It's not as bassy as Average_Joe says it is, either, IMO.
LS6: For me, the LS6 seemed to be the star of the Rooth lineup (your opinion may very well differ) --- very refined sounding; bass is only a tiny bit boosted, basically perfect for my tastes. The highs are not bad, but it does have a vestige of the TWFK-esque sound in that it's quite loud at the 6-8k range, so cymbals are still slightly too present and shimmery for a true "neutral" stance, but it's not overdone and very well-controlled. For others, it can be slightly boring, however, because of the flat behavior of the frequency response. It's one of the flatter-sounding CIEMs I've heard thus far. Yet, at the same time, that's what helps it stay in my ears for the longest time out of the entire lineup of CIEMs. I can imagine that the UM Miracle splits the difference between this and the LS8+, though I couldn't say for sure.
LS8+: It's deserving of the 'flagship' moniker, methinks. It takes the basic LS6 sound and adds some pizzaz to the low and high ends. It's more airy, and more hefty in the bass. Refined, yet sparkly at the same time, while keeping the detail of the LS6. In the end, it doesn't stay in my ears as much as the LS6 (call me boring, it's okay), but it's very nice, nevertheless. I believe that it has the most potential of "wowing" people on first listen, as it has a wide, airy soundstage that is almost reminiscent of those of electrostatics. Halo products should be that way, and I think Rooth did a good job in tuning it in a way that makes the sound exciting, yet refined at the same time.
LSX5: ...definitely the most 'excitement' oriented of the Rooth lineup. It takes the basic sound of the LS8+ and takes the U-shaped coloring even further. It's fun for the first 20 min. then it gets sort of annoying. It's quite a bit more sibilant than the LS8+, so that's why. Bass is big and dynamic. It doesn't feel bassy at first, but then you realize how bassy it can get. Perhaps I'm just too used to the BA sound.
LS8: ...surprisingly the most monitor-like of the bunch, though not quite the quintessential monitor sound. It's warm in the mids, and reminds me of my 4.A with a little more bass and slightly less lower midrange presence. It's very pleasant, but I think the left side has a partially-faulty DTEC that makes a weird buzzing/vibrating sound so I don't know if I'm hearing it at its full potential. Less sibilant than the LS8+ (though I wouldn't say that the LS8+ is necessarily sibilant), about the same as the LS6. Less clarity than both as well, but it's not really lacking in clarity, either. Decent transparency. Not really as airy as I'd hoped, which was surprising. Sort of like my 4.A. I dunno, might be because of that broken driver.
The LS8+ (left) and the LSX5 (right)