I'm a hypocrit, I admit, I didn't like the iphone 4 when I first saw a sneak peek at it. It's since grown on me. Even the round buttons don't bother me anymore (I held out the longest disliking htem).
I think functionality (like how well it fits in your hand) I stand by my statement that the old form was better as it curved into your hand. But, honestly, the new form isn't that bad and you quickly don't really notice it anymore anyways. So apparently functionality wise not that big a deal.
I think at this point I'd choose the new form over the old one, even though I have to put a case on it due to "fragileness" (ok, I've seen some reports that say it's not near as bad as this forum would have you think but since State Farm no longer insures cellphones I am not taking a chance). If nothing else it just wouldn't feel as much like a new phone if it didn't look different from the old phone to tell the truth and for that reason alone I'd pick the new one.
Though, you gotta wonder... I'm pretty sure Apple chose a glass back for the niftyness value (as well as thinness). Why not carbon fiber as that also has a niftyness factor, I'm pretty sure they could still stay thin (and light), and it has way more of a toughness factor than glass (or plastic)?