Seasons 1 and 2: wonderful.
Seasons 3 and 4: flawed but watchable, with a few great episodes here and there.
Season 5: mature and experimental in its finale.
The show was meant to have only two seasons. Fans loved it so much that they successfully mounted a campaign to extend it. Every season after that could have been the last, and it shows: the plot becomes wildly inconsistent, and there are mid-season mini-finales with episode 13 in both season 3 and 4, just in case the network decided to end support at that point.
So, practically the show has five finales:
S2E22, S3E13, S3E19, S4E13, S4E24.
What about the season 5 finale? Bittersweet and experimental. It’s Chuck and Sarah’s greatest challenge and greatest triumph.