I'm sure I would love Mission: Space if it weren't for my motion sickness issues. But with that problem, I won't even risk the "tame" side. Some days it doesn't take much, and no way I'm ruining an entire Disney day just for one ride.
I am a complete wuss when it comes to roller coasters, anything that will cause motion sickness, etc. DH tested out the "green" side of Mission:Space for me first, and convinced me that I would be able to handle it. Really, it's very tame, and I was fine on it (and I get queasy on the teacups even if they aren't spinning!). If you can handle Soarin', you can handle this. The only thing I would say is that if you have a headache, the "blast off" part at the very beginning puts minor pressure on your face, and that might cause problems if your head was already thumping! And no, the blast off isn't really a blast off, so again, no problems.
I found Dinosaur to be much more traumatic, and will never do that one again!
Time marches on. Eventually you realize it's doing it across your face.