October is great. Never considered it a weak/unfinished/whatever else album. It was great at the time it came out, and it's great still.
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I think that one of the possible reasons it's underrated by many fans is because many might not know it very well.
Take, for example, a 16 year old fan. Let's say they've been a fan since they were 10, that's 6 years ago, year 2002. By then U2 has released 10 studio albums, The Best Of, Rares, concert videos, you name it. That's a lot of music, a lot to choose from. You give something a listen once or twice, if it's not your favorite you have tons of other stuff to listen to.
Another thing is, U2 music changed a lot over the years, Zooropa lovers might find October not to their liking, and vice versa.
Well, anyway that's just my 5 cents on the topic.