Its simple, this is the last time U2 really challenged themselves and their listeners. I mean REALLY; some of he material on this album can be a bit tough to listen to (being at times very dark) but no less compelling, in fact compelling is one of a few perfect words to describe the album. Detractors of this album would have U2 doing either "Joshua Tree" (which I personally love btw) over and over or overly mainstream hit albums like the first two of this decade (I do like the most recent, though, "NLOTH").
U2 could not have chosena better time period in which to do an album with techno / electronica flavours either because the 90's electronica scene has yet to be bettered, across the board and U2 didn't just jump on some 'techno bandwagon' on POP, they had been progressively, since 1990, using more and more electronic techniques which culminated in the 1995 album "Original Soundtracks 1" (with Brian Eno, the band released this album as "Passengers"), the same year they started workon POP.
Every track is amazing, really. I'm a hard man to please with music and I genuinely rate POP in my personal top 3 favourite U2 albums at number 1. Its well worth a tenner of your hard earned. If you like your bands to be brave and to try something new musically then buy this. Also one other thing, lyrically this has the most words of any U2 album, its FULL to the brim with brilliant poetic (and at times very dark) lyrics by Bono, again his last best performance in that sense.