It's big. If you want to see anything, you need to sit either on the Arena or Level 1, relatively near the stage. I presume (and hope) that there will be screens for the upper level, but if you're content with that, you may as well buy the DVD.
I saw Carmen there in May, from pretty much the highest seat. The sound was good, but they didn't have any video, and I couldn't see who was singing what. I enjoyed it nonetheless, but I think this may have been helped because I've already seen Carmen a few times.
However, I'm still going to go!
Chris