Just got around to watching The Joker Express this weekend and thought it was ok, but nothing really special. I think the fact it was surrounded by so many superhero team up episodes couldn't help but make it seem a bit average. We've had Superman, The Flash and Green Arrow in recent weeks so a straightforward episode with Batman and co teaming up against the Joker solo was bound to seem a little underwhelming.
I agree this season so far hasn't really lived up to the excellence that was season four, but I'd still like to see this series continue, because I love Batman cartoons. I like the fact that the series is relying on different villains both from Batman and other DC superheroes rogues gallery as well, instead of relying solely on Joker and Penguin stories. Joker has only been featured once and the Penguin hasn't shown up at all so far, a welcome change of pace.