Watching it for the second time tonight, my opinion dropped a lot, and for me it's no where near as satisfying as "Justice".
Clark was, until about the 3/4 point in the show, perfectly willing to kill Chloe, until Lana talks him out of it. All the crap about how Clark inspires everyone at the end is just, well, crap.
Meanwhile, at the end when he decides there's no way Chloe's going to die, he has no idea how he's going to save her without killing everyone on the planet, with about twelve minutes to figure it out.
It would have made more sense if he was committed to saving Chloe all through the show, and he and Lana spent their time figuring out a plan, and he knew what he was going to do long before the last minute.
As it was, the only reason he didn't have to kill Chloe was because it happened to be Cosmic Boy and Lightning Lad that came back, not Bouncing Boy and Matter Eater Lad. Sheer coincidence and circumstance, or he would have had to kill her anyway. So his conviction doesn't really mean anything.
Now something I realized watching, there's no way that the Legion would be so personally familiar with Clark, to the point of calling him "Kal" like he was their best friend, if they hadn't met him before. Would I go back in time and expect to be on a first name basis like that with Charlemange and Roland, no matter how many miracles I was performing for them? If they come back in time and Clark is going by the name "Clark", then they would call him "Clark". It's a silly little nit, but their behaviour just doesn't seem right.
The details of Clark's history are a little too detailed, like his baseball, but I'll write that off to humour. But knowing the details of the exact date that Brainiac was destroyed? There is no record of that, it would only be what Clark decided to write down or speak of decades or even centuries later. It just wouldn't be that worth mentioning.
Moreover, the Legion wouldn't know anything about Brainiac that Clark didn't know, because what would be the source of the information? And how would Clark learn anything once Brainiac was destroyed? So how do they know that Brainiac's host being destroyed would even stop him?
Finally, Brainiac's master plan makes no sense. We've seen in past season what Brainiac would do if Clark never came to earth, and it isn't drain it of all knowledge and then let Doomsday destroy it. So the whole thing just seems made up for this ep, ignoring what happened the previous season.
This all stands out, and reading comments about how this is the best season ever, Smallville's finally on track, this is better than "Justice", etc don't ring true with this episode. It's a fun, easter egg filled episode with some cool guest stars, but it's hardly a "great" episode, and it's no more coherent than anything else in the series.