I hate riding Space Mountain - I just find it very turbulent/nauseating.
I hate the Sun Wheel at night - it terrifies me (I didn't know I was scared of heights until I tried it for the first time, and it happened to be night time.)
I tried Sun Wheel again a year later (our next trip back) in the daytime and was 'o.k' with it- not huge amounts of fun, but o.k.
I love BTMR and I LOVE California Screamin'! I was so terrified to try it for the first time, but did it without fuss to encourage our son, who also had never been on it, and who really wanted to try it.
Turns out, our then 6 year old loves it, too!! It's probably our favorite rides.
It is very smooth, and my advice is to turn your head/body into the turns. YES! Hold your head back when you take off. It is a cool feeling- I know what people mean when they say it takes your breath away, but it's a little less harsh than that... I mean, that's the only way I also know how to describe it, but it's a little more than that. Almost like a little thrill goes racing through your chest and then throat!
Good luck, and do only try it if you feel o.k. about it, but I think if I can do it, you can too. I had never been on any roller coaster before (BTMR and Space Mountain were the only ones.) My whole family does not like Space Mountain, we all find it to be more wild feeling than California Screamin', but I know that is opposite to what many others feel. I think it's because SM is in the dark, and I need to see/know where I am going. Same reason I didn't like the Mummy coaster at Universal (going backwards isn't much fun when you can't see where you're headed, LOL!)