Thanks to everyone who took the time to post about this! After carefully reading all the posts, I bought my ticket (about 2/3 of this thread was here when I bought it), and then I spent the next 1/3 of the thread second-guessing myself.
But I ended up being thrilled with my seat, which was BC23 (banquette). I felt I had a good view of everything. I only had to turn my head slightly to see what was happening at the rear entrance. As far as loot - I got a dumpling, a shaker, and a page of the book, which pretty much fell in my lap. I was also snowed on. I didn't realize letters were being handed out until the person next to me got one - I was looking somewhere totally different at the time - but I think if you wanted to get a letter, this seat has good potential for that, if you make eye contact when they're being passed out.
As far as interaction - a lot of action takes place in the BC23 area, and cast members would occasionally talk to me. I got lots of eye contact (especially with Denee Benton) and people singing directly to me. However, you're not dragged into the action, thank goodness. I don't have much to say about Groban interaction since I had no idea who he was before I started looking into attending this show, so I wasn't looking at him all that often and wasn't that clued in to where he was at any given moment. There was plenty of legroom (I'm really tall), and there was enough room for me to put my small bag next to me to my immediate right, without crowding the person to my right. However, you do have to keep your legs tucked in, since cast members occasionally use the vacant floor space in front of this seat, and I wouldn't recommend putting anything on the floor except your legs. Thanks again to everyone who posted - it was really helpful!