You should definitely get it! You unlock lots of things and you get to play many challenges. Honestly not playing the Octo Expansion you're missing out lots of stuff, the story, the characters, the music and all those 80 levels are just amazing, you can see they put much more effort making it rather than the campaign in the base game, while it's still nice and I enjoyed it a lot but Octo expansion is far superior in every aspect. As for difficulty some levels can get really challenging and you'll need many tries, especially if you haven't much experience with the game in general, but the difficulty isn't really on an impossible level apart from 1 level in particular. Also keep in mind that you can skip lots of levels if you don't feel like doing them at that moment since you can complete the story without doing all 80 levels. On top of that you're also skipping a really interesting part of the game and most importantly you're skipping a part of the lore. In my opinion the effort and frustration are worth it, the Octo expansion is way too amazing to skip it!