Underrated: KEF Space One (original wired version, haven't heard the latest model) Perhaps completely unrated is more accurate, as I haven't seen measurements or any review anywhere haha! (list price too high, but I paid only $99 on sale at newegg)
Rated just right: Sennheiser HD 58X ($150 and worth every penny and more)
Overrated: NAD Viso HP50 (list price too high, I paid $170 which I think is also too high, but maybe I'm just cheap, or my pair may be meh)
Compared to the KEF, the NAD have flabbier bass, that doesn't extend as deep, and there is a dip around 1k to 3k that profoundly impacts (in a bad way!) operatic soprano voices where the singer uses formant tuning to produce high second harmonics on high notes from high B and up. It probably goes unnoticed by most listeners because they don't listen to the small group of singers this applies to, such as Leontyne Price and Kathleen Battle, who almost sounds like a different person, the change is so profound on her high notes. (It can also be heard in how it weakens Mariah Carey's whistle notes, which some detractors might consider a benefit, I suppose lol. I happen to like her haha!) I did find that using some EQ, I was able to correct that dip pretty well. Oh, they also don't have the same dynamics as the KEF, but I think only listeners of high DR classical music will really notice or care. Some instruments do sound really nice on them, although I wonder if it's accurate.
The KEF have deeper bass, the bass is tighter, it doesn't muddy other frequencies the way the NAD sometimes does, the midrange and treble seem flatter (at least as good subjectively to my ears as my Sennheiser HD 58X). The Senns aren't as suitable for all genres as the KEF, mostly due to the bass. The KEF are fine with rock, metal, opera, baroque, chamber, choral, etc. The KEF are not as comfortable as the Senns, they are "over ear" but might be semi-on-ear for listeners with larger ears. I worry a bit about the wiring, as one pair of my KEF M400s have developed an iffy left side that cuts out occasionally if I move my head, and I've read a few similar user reports from others on that model. I like the way the Senns are wired much better. KEF needs to put their drivers in headphones with a better form factor, and with robust and better thought out wiring. Best to avoid running through the headband at all, and copy Senn on that, I think. The M400 and M500 are on-ear, which isn't really the best thing to me, and even the Space One is still a bit too small. If they'd put more thought into some areas they'd have a clear winner on their hands, I think. The Space One has active noise cancelling, by the way, and it doesn't ruin the music, but I leave it off anyway. I generally choose the Senns over the KEFs for most music, just due to long-term comfort (I did bend the headband on mine for a looser fit). In general, I'd still rather use loudspeakers due to less compromises, but I've done a lot of headphone listening in the past year.
Obviously, all of the above is just my own opinion, with my own musical tastes and preferences, and my specific ear shape and possible hearing defects. I'm getting a bit on in years, but I can still hear up to 15k quite well (not as well in the right ear, but that might be due to wax buildup and/or fluid in the inner ear--I have a few appointments in the next couple of weeks that will hopefully sort it all out!). I was very careful to always wear double hearing protection in my Navy years (Nuclear machinist's mate in the noisy reactor room on a carrier haha), and when doing anything around guns or loud equipment as I value my music too much to want to go deaf. Better dead than deaf for me, I couldn't live without my music.
