The plot is nothing special, but the humor and action are enough to carry this first episode. I really liked the interactions between Tora and Ushio. Ushio is a pretty typical 90s lead, but I did like how he was just going to leave Tora in basement rather than pulling out the spear for some dumbass reason. He doesn't do it until he's forced so he might have some modicum of sense. However, in the banter between Tora and Ushio that makes the episode. The two have an instant chemistry which makes the back and forth between them delightful. I like how arrogant Tora is only to be brought down to earth by the fact that he's always going to be trapped by the spear. Even after being freed he can't get away from Ushio because of that spear so if he doesn't want to get pinned then he can't go about his plans to drag everyone to hell.
Tora has an awesome design that's good for both comedy and being intimidating when that's called for. All the character designs are refreshingly retro and they're all very expressive. I like how flexible the retro art is because it's good for comedy and more serious moments. There is more detail in the faces and they're capable of lots of different exaggerated expression.
The action this episode was short but extremely well done. Ushio's transformation when he's using the spear is pretty badass looking.
The music was awesome this episode as well. There were a lot of different tracks and they fit the mood pretty well. Plus I can't help but love the cheesy rockin' 90s opening song. The ending song is a bit less memorable, but that opening is one of my favorites from the shows I've seen this season.
Other than that there's not a lot to comment on. This'll probably have a monster of the week format going forward. Tora is going to be attracting more demons and they'll have to fight them off together. There's some drama to be had because the spear changes the person that's using it so Ushio will have to worry about losing himself.
It seems like holding the spear and transforming gave Ushio a lesson on it's history so there was that awkward exposition dump at the end to explain the history, but it's not like we expect top notch writing from this. It at least sets some longer range stakes for the series.