It's a cute ride, but don't get too excited. TLM won't be knocking Pirates or HM off of anybody's favorite dark ride list.
In my case, based on seeing the ride-through video several times, and my love of the the songs, characters, settings, story, and relatively high "cuteness quotient" of the ride, I already KNOW I'll like it more than those!
And I'm sure it will be the favorite dark ride of many diehard Little Mermaid fans.
For me, it's more of a question of whether it can pass It's A Small World, Splash Mountain, Many Adventures Of Winnie the Pooh, Peter Pan's Flight, and the late great Snow White's Adventures which are on top of my MK darkrides rankings. If we are only counting current rides, I can't see it finishing any worse than 5th.
For my faves among all current attractions parkwide, I expect it to finish no lower than 8th, as when shows are included it will have to contend with the 4 current darkrides mentioned above, plus Country Bears, Tiki Birds, and Mickey's Philharmagic.
I'll have a better idea once I ride it, but based on my taste, I think it will probably finish somewhere between my 4th and 8th fave parkwide, and somewhere in my Top 5 if only counting current darkrides.
I expect it will make my Top 12 current WDW attractions when including the other 3 parks.