I should clarify my thoughts, here.
Regarding the JSAers, I'm accepting that it probably won't go well, but will watch because I want to see what they look like. What I saw in the preview had a strong 1940s movie sensibility to it, and that's something I've wanted to see for a long time. So I'm looking forward to it, even if the script is a complete turkey.
Will Geoff Johns do a good job? I don't know. He can be good, but he's too easily influenced by people around him. The first few issues of the JSA series reeked of James Robinson's "The Golden Age" influence, and once that (and he) went away, things got better quickly (except the lurching "the universe just vomited Hawkman back up so stop asking questions" stuff). Falling in with Alex Ross similarly produces weird obsessive plots that go nowhere, and his Green Lantern work reads like Alan Moore fan fiction. When he can find his own voice, he's good, but it remains to be seen if he can do that here.
(At DC, I'd like to see him in an editorial capacity. He's got a good eye for keeping details straight and knowing what the majority of the audience is willing to ignore when it changes. He's also not bad at coming up with big plot ideas, even when it overreaches his ability to write it.)
Regarding Smallville itself, I forget when I tuned out, but there was a very good season finale that basically ended with "everybody dies." It had a lot of potential to juggle things around and change the status quo, and I envisioned (for example) Lex and his father swapping bodies (a nod to the Ultra-Humanite and explainiing why Lex never seemed at all evil) and Chloe going into hiding and becoming Lois (before that awful actress was introduced). The first few episodes of the next season seemed to just be saying "just kidding!" over and over again. That's about when I just drifted away.
Since then, I watch if there's nothing else on, and it always gives me a headache. Elements seem hauled in and out at random, as opposed to keeping to an overall plot, and half the time it's like they're introducing things just for the sake of saying "look, we remember this character, too." On the one hand, they seem like they're moving too fast towards something (as you point out, he's 99.9% Superman in practice as "The Blur," and they seem to have introduced everybody in the DCU it's cheap to license), but on the other, it's painfully slow.
Then there's the fact that Clark is still the same whining, manipulative, adolescent creep (I'm generalizing the low points, but you get my meaning) as he was in high school, who needs a father figure to slap him around, some. In fact, the loss of Jonathan Kent specifically seems to have derailed the show, since there's no longer anybody to act as the kindly voice of reason. Everybody defers to him, rather, making his stupid choices and aggression seem like that's the ideal the writers are holding up.
It's actually kind of funny that the things that kept me watching are long gone and, really, had nothing to do with Superman. I thought the exploration of their (fake) Native American myths was interesting enough to go on for a while, as was the whole "Splendor in the Grass" bit with Jor-El having visited Earth in the past and the various Luthor connections. Actually, I wouldn't have minded if the show had been more about the town and its history than the brooding Neanderthal, now that I think about it.
OK, I think I'm done venting on the subject...