"Senjutsu" is a very good album. It's one of Iron Maiden's most complex and experimental recordings to date. Many of the band's casual fans (and a surprising number of their more devoted followers) don't seem to understand that the band has no desire to make "Number of the Beast Part 2". Since 1995, they've consistently been writing longer songs, with a heavier progressive rock influence. I grew up listening to the "classics" of progressive rock (Yes, King Crimson, Pink Floyd, etc.), and seeing Iron Maiden incorporate those influences has been exciting.
The album ends with three consecutive 10+ minute songs, solely written by founder and bassist Steve Harris. It's remarkable that these sprawling, complex songs were written by the same man who once wrote dumb, energetic rockers like "Prowler" and "Iron Maiden" on their debut. (I enjoy those tunes, of course, but I respect the fact that Harris is writing the songs that he wants to write, instead of trying to recapture the simplicity and energy from 40+ years ago). It's unclear if this is the band's final album; if so, "Hell on Earth" is a cathartic way to end their catalogue. (When the galloping abruptly begins, interrupting the long, melodic intro, it felt like seeing a friend one last time before they move away).
There are still some shorter, more energetic songs that might appeal to fans looking for another rendition of "The Trooper" (such as "Days of Future Past" and "Stratego"). These songs are solid, but not spectacular. The leadoff single, the folk-influenced "The Writing on the Wall", is the best of the shorter compositions, despite being somewhat slower. (With his lengthy solo on that song, Adrian Smith demonstrates why he's the best of the band's three guitarists).
There's some self-plagiarism on this album, but it's not as egregious as it was on "The Book of Souls" (where there were three songs that essentially copied-and-posted distinct riffs and melodies from previous albums). Many people have dismissed "Death of the Celts" as a re-write of "The Clansman". There are certainly some similarities - both are lengthy (9+ minute) songs about ancient British tribes preparing for war. The latter song, although long, is fairly simplistic, and has angry, defiant lyrics. "Death of the Celts" is far more technically complex, and is about accepting the inevitable defeat. The other song that comes closest to self-plagiarism is "The Time Machine", which doesn't quote any specific Maiden composition, but is an amalgamation of all the attributes you'd expect in a Gers/Harris composition.
A few things I disliked about "The Book of Souls" (and other recent albums) have been rectified here. First, there aren't any instances of the song's title being repeated ad naseum in the chorus (sometimes this works, but clunky choruses have ruined several strong songs). Second, this is Harris's most complex and energetic performance on the bass guitar since 1995's "The X Factor" (and represents a significant improvement over the past couple of albums). Third, despite the band stubbornly refused to down-tune their instruments, Bruce Dickinson often sings in a lower register here. The strained vocals that were noticeable on the past two albums are still present, but much more contained.
I don't pretend to be objective. Iron Maiden is my favourite band, and I've liked every album they've released during their progressive "reunion" phase (2000 onwards). I'm not sure where I'll ultimately rank this release in their 17-album catalogue. The band, despite being well into their sixties at this point, have proven they still have fresh, new ideas. I hope there will be an 18th album but, if not, this would be a fitting end to their career.