Doesn't surprise me to learn that this one bombed. I mean, I was a dumb eight-year-old in 2001, and even I wasn't especially interested in seeing it. And yet, there must've been something about the title and character design that made an impression on me, because here I was, a quarter century later, still kind of curious about what I was missing.
And... I actually quite enjoyed it. My younger self would've been pleasantly surprised. There's a Y2K feel to the animation, the upbeat R&B soundtrack (god, I miss this kind of music in movies), and the general sense of humour that hit my nostalgia bone, and I was really into the premise and worldbuilding. Just the concept of making characters out of human cells and organisms is genuinely clever (it's very Inside Out meets Magic School Bus), to such a degree that I'd have to imagine that the jokes and conflicts basically wrote themselves.
There's also some live-action stuff going on here, and while it's definitely on the lazy and lethargic side (especially given the over-reliance on gross-out humour, and the fact that Bill Murray's turning in another one of his "I'm way too good for this" performances), I still found it funny and energetic in a way that gels with the animation. But you might want to take that opinion with a few grains of salt - I'm a sucker for any movie that combines these two mediums.
Grade: B+