Finishing up this series now. I really enjoy it, though there are a few episodes I don't care for (most anything with Livewire, she really bugs me). The voice acting is great and they do some cool things with Superman's rogue's gallery. They also have team ups. We meet the Flash, Dr. Fate and Inza, one version of Green Lantern, Orion and many of the New Gods and there's a couple of meetings with the nut from Gotham. I've probably forgotten a few as well. This what really sets up things for the Justice League series, far more than anything they did with Batman. Oh yeah, the Legion of Superheroes makes an appearance as well. Damn time travel stories!
The commentary is also great on the 6 or so episodes they chose to include it on. That really should go without saying. They talk about how they de-powered Superman in the earlier episodes. They sort of address this in the last episode of JLU, but I also think the more power he uses the more he needs to recharge. So he doesn't need much power against, say, the Toyman, but when he's fighting Darkseid, then he has to recharge a lot longer and he's weaker until then.
Oh, and we also get to meet Supergirl. There's a bit more humor in this series, but there's also pathos, as in Dan Turpin. I remember seeing that episode when it aired and what happened sure caught me off guard. More proof that Darkseid is a douche. It's still shocking some 18 years later.