After finally seeing Superman Returns, my comments:
X-Men I was passable. X-Men II was passably good. And while no superhero movie can ever be perfect, as the main premise behind any superhero is, in the end illogical and irreconcilable, Superman Returns actually makes that point moot by being something better than perfect...
watchable.
It's a pretty good movie. It sucks you in well enough that it achieves suspension of disbelief... although for anyone who's read "Man of Steel, Woman of Kleenex", Lois Lane's kid will stick out as something pretty unbelieveable.
Well, the story is slow in some bits, pretty predictable at others, decent enough, but it doesn't really matter.
What matters is that Superman comes off as believeable, with very few over-glossy CGI moments and relatively realistic physics. The world looked and felt real, much in the same way Sam Raimi makes the world of Spiderman feel real (except for the crappy CGI), and not a parodic caricature of the world, as seen in all of the Batman films (including Batman Begins... it's a good movie, but the world is too... goth...

)
The main characters are well-enough developed and human, although Jimmy Olsen and Perry White come off as pretty two dimensional, as does Lois's boyfriend (whossisnameagain?).
Overall, I'd rate it as four stars out of five, and worth more than one watch.
PS: If you've never read "Man of Steel, Woman of Kleenex", I'm pretty sure some fanboy will dig it up and post it on the net soon enough. It's a good read.