Reviews on progressives is sort of useless from a "this episode was good this one was bad" perspective. However, as has already been said, discussion of a series as it progresses can be interesting. LOST is a good example of this.
The traditional review would make more sense for either long-running progressives like say, Supernatural or X-Files, where a new viewer is not necessarily best served starting at the pilot, and the meta story is sometimes not addressed at all in some episodes that can be effectively used as an introduction. Or, as was said before, anything where there is no meta-story, like a sitcom, where being able to learn the best season or best episodes becomes valuable.
The inverse of that would be a review system that can be used to create a 'skip list', where a set of episodes are deemed a combination of poor/unnecessary and can be safely skipped without diminishing the series' experience. I believe Danielle Riendeau has one of those floating around for FarScape.
So short answer; I think any good review is valuable. TV show episodes are no exception. The key is they need to be written in such a way as to help both fans and newcomers to their respective series.