There have been rumors about a Trigun movie for ages now, and I've heard everything about it, from it being the anime condensed into a movie, to an unrelated storyline, to something that follows the manga. I'd really like to see a Trigun animation that follows the manga--so desperately do I want it! The series followed the manga pretty closely, but since the manga wasn't complete when the anime was produced (as it usually goes...), the story deviated with a cop out ending.
I am also glad that the Hellsing OVAs are being made. The series followed the first two volumes of manga, because that's all that had been released when the series was produced. Interesting where they took the story, but it really isn't Hellsing at all. The storyline is completely foreign.
Don't even get me started on FMA, though. I haven't read the manga, but know a lot about it. Quite an incredible story in the anime, but like Hellsing, a complete departure from the actual story (from what I hear).
You really have to give the writers/producers credit, though. Because a manga becomes so popular so quickly that there's a demand for an anime, they work extra hard to pull some incredible ending out of their asses just to appease fans. That takes some serious talent, to blend the known storyline seamlessly with something completely new and different from the direction the manga-ka is wanting to take.
In the end though, yeah, I wish they would stop screwing with storylines. There's got to be a better way.
On a side note, some animes follow the manga perfectly. Fruits Basket, for example, follows the manga very well (even though it kinda mixed some chapters up) for the first 7 volumes or so, and the place where they stopped the series was a good stopping point. (Even though, imho, they should continue the series to the end of the manga, and since they didn't eff it up in the first place, they could do that.)