I discovered this forum the same way that Jonnie above me did. I was dumb and didn't think to search for anyone else's experience with TuneCore until after paying them and waiting for the results, since I had been recommended to it by a podcaster I liked. Naturally, reading this thread had me really worried.
However, my results varied DRASTICALLY from what this thread has reported, and I think that's due to the type of music I make. It's poppier and appeals to 16-24 year olds evidently, as we scored an 80% on one song and 51% on another (this track was much less poppy, was a ballad, and was occasionally classified as "boring" by some of the younger audience but praised highly by older listeners, similar to the other two posters on this thread).
While there were plenty of people just explaining what they heard sound for sound and giving it somewhat empty praise, there were more than a few reviews that gave suggestions that were helpful, such as saying they wanted the singer to be louder in the mix, indicating whether they thought the song should be longer or shorter, stating their favorite part of the song (really helpful for promo teaser clips), etc. My band mate and I still received the occasional troll comment ("this sucks, I want to die listening to this and I'm not going to tell you what about the song makes me feel that way"), but TuneCore also gives you a weighted score based on how relevant the review is and the reviewer's rating history, apparently, so it helps even the score out if you end up getting troll reviews. Either way, at least in our case, it was abundantly clear that the reviewers were talking about our specific songs.
My disappointment was in the lack of data. The only really useful demographic they give you is age range, but they don't say anything about race, geographic location, or even the listener's favorite genre of music or other artists they like. Also, the age ranges are a little wide on the lower end: 25-34 and 35-44 seem fine, but 16-24 seems too distant, especially when 70% of the reviews come from that age range. That and the fact that you probably couldn't even pay 24 year old me to listen to anything I liked when I was 16.
Overall, the reviews were really useful and we've already started using some of the verbage in the comments to describe our music to others - press, potential gigs, potential fans, you name it - sometimes it's helpful to have an outsider opinion. Where we look at our work and say "there's no possible way to fit this into one box!", others can listen and immediately check off where they think it fits. I was freaked out after reading about the lyrics comment from OP (that is really messed up), but luckily we didn't have any of those issues. Probably wouldn't splurge for a higher tier of listeners, but if $15 is all I have to pay for marketing research then I'll take it.