CG always looks way worse in trailers than in the final product. It was top notch in Prometheus, so I'd give it the benefit of the doubt.
Eh, if that was the case then they'd be really stupid for making their trailer look worse than the movie since that's supposed to bring people in.
If anything the reason you can ignore the cgi in the movie itself is because you'd be caught up in the story so don't focus on all the details.
Personally for me it's an instant no if there's bad cgi which is the case for this movie (story doesn't seem interesting either to me tho although that'd be okay if it was actually scary somehow)