Osmosis Jones is an energetic, imaginative, and inventive film in ways different from other Farrelly films (Dumb and Dumber, There's Something About Mary) but yet retaining their trademark low-brow stamp.
The primary protagonists in the film are Frank (Bill Murray) and his body, the latter seen as an organic city complete with a police force (representing the immune system) and a mayor (in charge of the brain). Frank is a total slob who abuses his body, not heeding the admonitions of Shane (Elena Franklin), his daughter. His body feels her pain: maintenance must be performed regularly and heck, a local mafia consisting of fungal cells has taken root in his armpit.
In the spotlight in Frank (the city) is Osmosis Jones (Chris Rock), a white blood cell, who is over-eager and consequently ends up in sticky situations that make him highly unpopular. When Frank eats an egg that has fallen to the ground (after he takes it away from the mouth of a monkey), a deadly virus, Thrax (Lawrence Fishburne), enters his body with the goal of killing Frank in record time. It's up to Osmosis Jones to stop Thrax before Frank is destroyed. Not only does Jones have to contend with Thrax, he also has to deal with a corrupt mayor (William Shatner) who has let Frank go to waste. Assisting him is a newly arrived cough pill, Drix (David Hyde Pierce), who goes beyond the call of duty (I'd kill for such side effects).
Murray is hilarious and does the job of a slob really well. Elena Franklin is also pretty good, without being overly cute. The voices, which are key to making any animation work, are done well.
From a scientific perspective, Osmosis Jones has the problem that the human body (nor a virus) does not behave like a monolith. The brain doesn't control the rest of the body any more than the anus does. I don't expect scientific accuracy necessarily, but I make this point to illustrate the top-down flawed nature of human thinking: to perceive everything in terms of categories and hierarchies (hey, so this movie did have some pseudointellectual content for me).
Aside from that, the movie is very clever in the way it portrays the internal workings of the body. It was amusing to note that a zit pulses and hurts because there's a party going on inside. There's a lot of groan (or vomit, if you prefer) inducing scenes, but most of it has to do with Frank's antics rather than what happens in his body (since it's all animation, it never gets that gross). By the way, there's another The Matrix reference.
The best and funniest scenes in the film involve Frank's inelegant ejection of a two-week old oyster eaten by him, prompted by Osmosis Jones' vigilance against an antigen (this is shown as a flash back). This is gag typical of the Farrelly brothers, who generally are inventive about how far they will go to gross you out. However, Osmosis Jones as a film is more interesting for its imaginative depiction of the internal workings of the human body. I recommend catching it on the big screen.