In the wake of Peter Jackson's incredibly successful Tolkien series, this movie tends to get a lot of flak. Yet in some regards, I actually prefer this version, and I'll explain why:
The difference of opinion is basically generational and dependent on what the viewer is looking for. If you are hooked on stunning visuals and "epic" proportions in every estimable regard, there is no denying that Peter Jackson's films are better.
While this film deviates from the plot in several instances--no doubt a consequence of condensing so much material into an hour-and-a-half--it does maintain some of the better quotes from the books; keep in mind that these lines are delivered in the style in which they were written, not watered down the way some of the most powerful quotes are in more modern versions.
Combine this with a cast of amazing voice actors (Brother Theodore is the best, creepiest Gollum, hands down; Paul Frees orc voices are chilling; Roddy McDowall and Orson Bean do incredible things; and, of course, John Huston; I am not familiar with the actor that plays Denethor, but I love that performance as well) and you've got what is basically an Elizabethan drama with watercolor backgrounds and animation.
The other major reason why people dislike this film, and again it was a creative choice, is the inclusion of songs. Peter Jackson made films for adults; these animated films are intended for children. I admit that the ratio of song to plot can get tedious in this film, but the reasoning is noble. If you've ever read The Hobbit or The Lord of the Rings, you know it is absolutely packed with poetry. I am sure it was this film's intent to preserve this feeling while at the same time emulating the musical style which has been popular with children's programming for years.
In conclusion, people often criticize this film on matters of taste rather than actual merit. If you enjoy animation and well-written dialogue, this is definitely worth a look.