I can't imagine the movie will be the same experience as the book, at best it will be a different kind of fun. The book goes into a lot of fun with science lectures that I don't think will translate well. I really loved the book, I'd say it's worth a shot.
I've wondered the same thing. The first trailer has a bit of voiceover that's directly quoting the book which sort of makes sense given that he's writing everything in his journal/log. My guess is that it will alternate between him talking as a narrator and talking on-screen given the context of the event.
I'm six chapters in and I like it so far. The jump from Watney's journal to the characters at NASA was unexpected. I'm actually thinking about skipping those chapters altogether because I like how how isolated things feel from Watney's perspective. Though, I'm afraid I'll miss important points if I do.
There was a point in the book where I had just read a long section from the nasa perspective and found myself wishing it would just go back to Watney only like the first part of the book had been. A few pages later I got my wish for some time because his comms went down. I think a little less on the nasa parts would be fine but overall they're a necessary part of the narrative.