First of all, I want to you know up front that I am NOT a rollercoaster fan. So take that into consideration as you read this. I absolutely hate big hills/dips and the way it makes me feel. I enjoy rollercoasters that go side-to-side a lot, like BTMR and Matterhorn (at DL).
I checked on this very board for reassurance about this ride before I decided to try it myself. Everyone here played a large part in my deciding to give this coaster a try.
I haven't been to WDW in about 4-1/2 yrs., so my memories are a little old, but here is what I remember. The 1st time I rode RnR I was so scared that I closed my eyes and screamed the whole time. By the time I got off the ride my mouth was so dry I could barely swallow.

I also felt a little lightheaded (probably from screaming so much). My DH asked me if I had liked it. And I honestly couldn't say. I knew I hadn't disliked it, but did I like it? So i decided to try it again. The 2nd time I was much less scared, since I already knew what to expect. I kept my eyes open and only screamed from the exhileration and fun of it. When I got off from my 2nd ride I knew that I had totally enjoyed it! It was a blast!
Are there any big hills? No. Definitely not or I wouldn't have liked it at all.
Can you feel going upside down? That's a tougher one to answer. The first time I rode it I have no idea what was going on. I couldn't feel anything specific, but I don't know if that's because I had my eyes closed or not. The 2nd time i rode it, I kinda, sorta could tell I was upside down, but by the time my brain had really registered it, we were already on to something else and no longer upside down. The only thing I could definitely feel was the corkscrew, but between going upside down and the corkscrew I was so turned around i didn't know which way was up most of the time until the ride ended!
Is it totally dark inside? Not as dark as SM, but if you aren't familiar with that ride then let's say it's not completely dark. You can see hints of the track from the neon lights that flash around. And you can see the road signs and such that they have up as part of the design, so it's not like riding in complete blackness.
The take off is a blast... literally. I was really afraid of how that would feel but it just turned out to be exhilerating.
Just the fact that you are considering riding it means you must like something about coasters, right? Depending on what you enjoy about them and what you don't like about them will probably determine whether or not you like RnR.
I totally recommend giving RnR a try. It's a very short ride, something like 90 seconds I think. So if you don't like it, at least you won't be on it for long, right? But I think you will like it.