Of the final four episodes, this is the weakest, which isn't saying much.
Owl's Well That Ends Well is an episode that continues with the emotional resonance of the previous episode, but it does make some compromises in the process. Spike's a bit annoying, and Twilight's nagging tendencies seem to be exaggerated. Still, it's far from a bad episode.
This episode has often been noted for showing Spike at his worst. This is indeed true. Spike's initial clamouring for attention isn't ever really addressed, and it doesn't come off very well on him. However, his later jealousy is the core of the episode, and is revealing about his character. We see how much Spike cares about Twilight through his worry that she would replace him. The importance of the #1 assistance title is probably due to his own insecurity, which is why he thought he could be replaced, but also because he values Twilight's approval. This added depth for Spike is the episode's strongest point.
The humour often works as well. There's a number of neat visual gags, in the form of clever cuts and little animation details. On that note, the episode looks great. One scene with Twilight writing at night shows some ambitious use of shadows, at least for this usually bright show. It's a very impressive effect and it looks phenomenal.
Perhaps Spike's attitude can get a bit grating, though. Sure, it comes from a place of character depth, and it allows for a lot of humour, but it's still him acting very badly. Maybe that's reaching a bit. Yes, Spike's actions are shallow and selfish, but that works more in service of his character than anything. Spike hasn't done a lot of maturing yet. He's still young. He's gonna act in ways that are immature, and yes, selfish. That said, that doesn't mean you have to like it. It can still be grating for kids to act petty, and though I don't think it gets too bad in this episode, I can see why some might. Twilight herself is also a bit too much of a nag here. I wish it were addressed a bit more at the end, but at least she acknowledges it and notes it as a problem.
Owl's Well That Ends Well is a great episode. It supplies a lot of depth for Spike, and it has some good humour to boot. In some ways, it's rough around the edges, sure. But it's still possessed of more depth than most of the season, and for that it's great.