I didn't think much of it.
- I wasn't so impressed with it.
- It wasn't so impressive.
- It didn't make much of an impact on me.
- How did you find the last movie by Tarantino?
- Well, I don't think much of it.
I wouldn't know.
informal- I have strong opinions but I wouldn't know how to express them in dance.
- If I said something that offended one of my students and they didn't tell me, I wouldn't know.