I saw this when I was 12 years old. I am now a retired physician and I think I enjoy it just as much as when I was a kid. Everyone talks about the special effects and they are absolutely great... the effects hold up.
The acting is typical 60s style but that's okay... it offers a wonderful sense of that era.
I think it would be a great movie for middle school kids to see in anatomy class because it is extremely accurate with regards to the human anatomy.
It's quite a ride! Five stars for me.