If you watch the original you probably won't enjoy the movie as much.
At the same time, you might not understand the movie as well as you could have. Very important things are noted but are glossed over, leading to a very confusing experience.
The anime and the movie follow a different story with different characters and different portrayals of characters we already know, which feels off. Not necessarily a bad choice, but imagine biting into what appears to be a chocolate cake, but it taste like strawberry. Not bad, but just... weird.
I watched both. Loved the anime. The movie was watchable and fairly enjoyable, but moments of questionable acting and plot reasons ruined a few portions of it. The soundtrack was great and there were excellent camera shots that brought a certain life to the film. But again, certain acting moments and plot reasons take away from the grand experience.
In comparison, the anime is great but also falls into it's own little holes. It is relatively short, sitting at 37(?) episodes, but it could've been shorter (imo). Still a good watch though. Worth it.
...
... Yeah.
Man I should really work on the drawings I promised to make for people instead of writing reviews for Death Note...
I'm not good at time management