I have a bit of a blind spot for this, I can quite see that some of the sequences are a bit of a bodge, or even rather poor (Lucy in the Sky with Diamonds is especially lacklustre) but I love the characters, the scenario and the imagination on display.
The versions of The Beatles here are far more appealing than the real deal (as evidenced by their appearance in the closing minutes), although to be fair Ringo seems pretty consistent (and it's rather touching when he offers to take Jeremy, the Nowhere Man, somewhere). He certainly has a great line in dreadful puns. The Blue Meanies, their "nasty medicine" quaffing Chief especially ("My dear friend... let us not forget that heaven is blue"), are deranged, hilarious and unnerving. Meanwhile Dick Emery's Jeremy Hilary Boob Ph.D is battily charming.
The film is at its best when it takes full advantage of the psychedelic flights of fantasy that the concept offers. The Sea of Holes, with its false perspectives and comic trickery (Ringo removes a hole and puts it in his pocket), and the Vacuum Monster (it sucks in the entire background) make particularly good use of the medium.