Most of the time, anime converted from the manga are pretty true besides small bits here and there. However, there are some series where the anime is much shorter than the manga, and it can also be the other way round. For example, take a look at Hellsing. The manga is damn long but the anime is only 13 episodes.
Same with Fullmetal Alchemist, the manga is still going on whereas the anime has finished at 51 episodes.
an example of something to the same effect is the case of Ikki Tousen. the manga (called "Battle Vixens" cause Tokyopop F'd that up) which came first, is around 8 volumes long but the anime, released by Geneon i belive, is only 4 DVD volumes long.
in my opinion, the anime sux but thats cause im a bit of a "pureist" even tho i dont know what to call myself. i think whichever came first, be it anime or manga, is the best. for example i think the Neon Genisis Evangelion anime is way better than the manga, but the manga for the Ikki Tousen thing is way way way better than the anime.
granted i do make exceptions. i like both the anime and the manga forms of Candidate For Goddess (or Pilot Candidate) about equal. i like the anime better than the manga for Trigun, despite the fact that the manga came first. and i also like the manga form of Gundam Wing, Endless Waltz a bit better than the anime which came first.