While the anime stayed fairly close to the manga, this live action movie makes substantial character and plot changes. For instance, Bunta is now perceived more as a drunken womanizer more than the retired legendary (yet still smug and egotistical) racer from Akina. Ryousuke and Keisuke Takahashi are essentially one character. Several other characters don't even appear in the movie.
The plot was also condensed considerably. Instead of being a movie covering a few races from the anime's first season, it compresses everything from the first three stages (which took the anime 39 thirty-minute episodes and its own two-hour movie). Obviously with about two hours of time, this movie had to drop several races and increase the pace of character and car driving technique development.
However, the movie does get some things right. Character development is focused on Takumi and his indecisiveness of what he wants to be in life. The actual car racing uses the real cars that the characters drive in the series, and most of the driving is more realistic than completely special effects. The mood of the movie fits, and it follows the general "teenager who has special skills but doesn't know it" plot of the series. Itsuki is still the overzealous racer wanna-be, even if his character's background was totally changed.
One thing to note is that there is absolutely no EuroBeat music in this version, which can be a good or bad thing depending if you're a fan. Most of the Chinese pop music is from Jay Chou, who plays Takumi.
In the end, if you're a previous fan of the series, don't go in expecting to see a live-action version of the anime or manga. You'll just end up disappointed that the movie tries to cover so much ground in such a short amount of time. This movie is like the later Harry Potter movies in that it is an okay adaption of a much better and more detailed story.