A movie, if done correctly, would be fantastic. Anything to make up for the TV show. With a larger budget, better CGI and special effects (cough*VisserMcMuppet*cough*) and no random non-existent plot elements (can you say disc?), the movie could prove to be interesting.
It would have the potential to draw in crowds of dedicated fans and others purely out of curiosity. The only thing I'm trying to figure out is how they would pull all this together into a coherent 2 hour film. I know they wouldn't try to pull all 54 books into one movie, that would be dumb.
Harry Potter is one film per book, and they can't do that. Films like Series of Unfortunate Events and the upcoming Cirque Du Freak are all based on books 1 through 3 of their respective series. Then again, those series are only about 10 to 12 books long. However, the Animorphs books are a lot thinner than those ranging around 150 pages, I think. The others mentioned can go up to 200 and beyond.
I guess Animorphs books don't have too many major plot elements. Perhaps they could at least go up through book 4 where all the major characters are introduced and focus on the plots that really drive the books forward. And of course there would have to be scenes of the kids being kids. School, dealing with family and all that.
As long as they stick to the basic rules:
1) No Andalite arm wrestling! I'm sure that's beneath them or something.
2) No Visser McMuppet
3) Don't make it look like Ax and the Visser are perpetually in morph. I'm sure they're not exempt from the time limit.
4) Don't make it look like the Andalites skinned Grover and Cookie Monster and turned them into fuzzy arm warmers.
5) Don't let the main characters look like college rejects perpetually caught in a High School time loop.
6) Where are the bloody Taxxons?!
7) I'm quite sure the Yeerks conquered more than one Hork-Bajir. "Hey! It's my turn to be the Hork-Bajir host!"