Well I know my reaction is definitely to be scared to death, as Cowboy Bebop is one of my top favorite anime series. A lot of people have had the same reaction, and it looks like people in Japan aren't any different. AnimeVice has put together some of the responses from their 2ch message board and here is what some people have said on the matter:

"Is Keanu Reeves going to grow shaggy hair?" "I've come to the conclusion that [making a Bebop film] is unnecessary and impossible." "I wonder if the famously short-legged Keanu can play the role of Spike?" "My only worry is Ed." "But Keanu doing it will be fun..." "The question is, who will play Ed?" "I wonder if Ein will appear?" "I think the anime movie is better." "Please don't turn Spike into a modern high school student LOL" "The Japanese staff is involved? And instead of C-grade actors they got Keanu? This could really boost the image of Japanese animation!"

As you can see there's a lot of worry, but at least a bit of the response is positive, though there are still a lot of questions to be answered. My main questions are if Keanu's playing Spike, who the heck is playing Jet, Faye and Ed? At least they can't screw up Ein unless they somehow forget he's a Corgi.