I did and the only thing I remember of the whole thing was the cool way the Magic Mirror on the wall was portrayed. Nothing else!
I don't think Ledger would have been big enough. Maybe I'm wrong but I always got the sense that he was more of a normal sized guy while Hemsworth is more on the huge-ish side. But, yeah, Chris Hemsworth is basically Sam Worthington if you lacquered up Worthington with several extra layers of charm. Worthington is more of a raw tough guy in the Steven Segal/Jean Claude Van Damme category. Hemsworth, I'm not even sure there's a category to describe how versatile he is as far as balancing strength, charm, vulnerability, & humor. (OK, maybe David Boreanaz & Nathan Fillion have come close but that's it.) Personally, I kinda wanted to see Hemsworth play James Kirk instead of George Kirk in Star Trek. I think Hemsworth bears a closer resemblance to Shatner than Chris Pine does.
I did, at the midnight premiere, just 'cause I happened to get off from work in time & it was free. Go me.