I wasn't expecting to like the movie all that much, based on what I'd heard, but fortunately, I was completely and utterly blown away and amazed by what I saw. From the very first opening voiceover of the late Marlon Brando, to the classic John Williams' Superman theme- just, wow. I'm not even a huge Superman fan, but I couldn't help but be overwhelmed with a sense of the iconic enormity of the character, and that I was- after so long- seeing him on the big screen again.
I'd heard that Kevin Spacey was a horrible, scenery-chewing, campy Lex Luthor; in my opinion, he nailed the role, in a way that even Gene Hackman didn't. Hackman- particularly in the second and later films- began to verge on caricature. Spacey played it straight all the way through (well, up until his outburst at the end, though that was frankly in keeping with the Luthor-from-film character). Someone here said Spacey wasn't scary enough- I'd have to disagree. Spacey's Luthor was intelligent and plotting, and I thought he was pretty dang menacing in the scene with Lois, where he was making thinly veiled threats against her and her son.
The rehash of the "real estate" scheme from the first Superman was, admittedly, a little bit of a letdown- I was expecting something bigger from the looting of the Fortress of Solitude and Kryptonian technology. Still, though, it shows consistency of character. I was just hoping Supes would say something about "not being able to teach an old dog new tricks," although it occurs to me he's used that line before on Luthor in the movies.
(Speaking of consistency of character- Luthor really needs to make a better selection in his choice of molls. First Eve, now Kitty. He needs to find someone with the backbone to stomach his megalomaniac schemes in the future. At least he finally ditched Otis.

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I thought that Parker Posey did a really good job of walking the line between bad camp and drama with her character.
Routh, I'd read, was basically described as imitating Reeves' performance, which I wasn't looking forward to. Again, though, pleasantly disappointed. He really made the performance his own, I felt. Definitely a worthy heir to the title of Superman/Clark Kent (although I couldn't help but think, during the sea plane rescue, that James Marsden might not have been a good choice as well.)
Overall, I think everyone did an excellent job- casting was superb. There was a suitable amount of chemistry between Bosworth's Lane and Routh's Superman (with a noticeable lack thereof between Lane and Kent, which is as it should be). I did think the kid's haircut was annoying, but if that's the worst complaint one can raise, someone's doing a good job.
Another scene that I really enjoyed was the Superman/Lane flying scene, with "Can You Read My Mind" playing in the background.
I had figured the kid would turn out to be Kent's, and I do find the "resolution" (such as it was) to be disappointing- what? He can't even raise his own son? Admittedly, I think there are some real complications with such a scenario- Lane's current relationship with Richard being only one of several- and, as my girlfriend pointed out, it is very likely something that will be dealt with in future installments.
I do think that the movie could have been shorter- if they did shave 20 minutes or so off, I wonder if they selected the best 20 minutes to cut. The "is he alive?" bit at the end seemed excessively drawn out. Again, though, a minor complaint when held up to the rest of the movie.
One thing that really struck me, though- during the scene with Lex and his thugs on Krypton Island, I couldn't help but think, "if only Batman were here now." Patrick Bale should have flown in on the Batwing, and jumped in there to take out the thugs. "Looks like you needed a hand." Then, when it was just Supes and Batman standing there facing Luthor, Superman would have said, "Do you mind?" as Batman shook his head. "By all means." And then Routh would have laid Lex out flat.
They really should do a Superman/Batman crossover film at some point. Doesn't have to be in continuity with the movies, but it could be huge. I can't imagine it not being a no-brainer to studio execs.
One last thought- will Krypton island continue to grow now? Or will it just float around out there as a one big kryptonite infested chunk of rock, screwing up gravitational patterns? The world may never know.