I just love Mission:Space, it is maybe my favorite attraction at WDW, although I can't say that honestly when I compare the number of rides on it and other attractions. Let's say that right now, it is my MUST-SEE, MUST-DO attraction each and every day. It thrills me, it amazes me, it inspires me. It is not just the G-forces. It is the beauty of the pabillion, the excitment of the planetary plaza music (especially at night!), the "space" feeling of the queue line, the pre show spiel... I think this is very well done, it builds the anticipation as no ride has ever done before.
The boarding is a bit disappointing but lasts 3 seconds. then you're in a spacecraft and you forget everything. The theming is the absolute best for a ride vehicle, almost as good as the ride itself. You feel really there. There is an amazing amount of details around you and you'll have to ride several times to see it all (and remember there are 4 positions): you have the sound effects in real time, the capcom-chat all around, the lights, the buttons that make sounds and have an effect on others, the panels in front of you, the video screens on the sides with real-time data and videos of what you are doing during the flight, where you are, dialogue with Capcom etc...
Then there is the ride, the uniqueness of the thrills, the fact that they last so long, the "almost 3D" visuals, the scenery itself during the ride, the "interactivity": be sure to check out the video screens on the side when you push a button to see an animation of what exactly happens!! I just feel good in this pod, nothing else outside exists, I am in space. Once again the interactivity makes you want to ride at least 4 times to test all the positions and you can build your own story... I was there with 2 friends during our last holidays and we really played our jobs, which made those the greatest ride moments of our trip. Nowhere else can you be as "active" as on this ride and play along.
At last, there is the post show, and while I didn't do everything there, I think the feeling is cool and busy as it should be, very technical, futuristic... the exit corridor is blah, but the song played, Destiny, is really good.
So all in all, no, I don't like it has replaced Horizons, which looked truly great, but in itself, it is to me a tremendous attraction which needs to be tested at least once. It appears that almost everyone not liking the rides are the one that have been sick on it, and I don't find this truly objective as a reason. It shouldn't ruin other's curiosity to try it.