Probably Inuyasha the Final Act would not be very enjoyable for those who have not seen the original series, but it does provide fans of the original series with a satisfactory ending to the story.
While I thought that the original series was quite good in many respects, I felt that the lack of a decent ending was unacceptable. Since they have now given us the ending, I have gone back and revised my original review and increased the rating to reflect this.
Some random spoiler-ish thoughts about the ending are below. If you haven’t seen the series, you are advised to stop reading now.
The most surprising thing about the ending is that Naraku turns out not to be the ultimate villain. He’s actually more of a pawn.
It turns out (if it wasn’t obvious already) that Inuyasha is not really the main character. The primary story is really about the damnation and ultimate redemption of Kikyou, with parallel redemptive stories involving Kohaku and Kagura.
Kikyou’s original plan to use the Shikon no Tama to make Inuyasha fully human could never have worked out. In fact her decision to abandon her duty and use the Shikon no Tama for a selfish purpose is probably what doomed her in the first place.
Kikyou’s abandonment of her duty may help explain Kagome’s behavior. As the granddaughter of a priest, Kagome would have been expected to serve as a miko on numerous occasions (at least whenever the shrine had a festival) but it appears that she refused to do so. She spends most of the story trying to avoid accepting the role of a miko, which is probably one of the reasons why Kikyou can’t stand her.
As we know from the beginning, Kagome is the reincarnation of Kikyou. Kikyou in turn is probably the reincarnation of Midoriko (Kikyou says this pretty explicitly at one point.) Furthermore, Midoriko was probably the reincarnation of Tsukiyomi–the parallels between her life and the other 3 are too obvious to ignore. So I guess she needed 4 lifetimes to get it right.
This explains why Kagome and Kikyou have a special connection to the Shikon no Tama: part of their soul is inside it. (And if they aren’t careful, the rest will be as well.)