Can't say as I'm terribly surprised. I stopped watching the series shortly after the second year, when SD-66 got taken down. It just seemed like everything else was going to be anti-climax. But I know a lot of folk who love this show, and will be disappointed by the news.
I do understand from some viewers that the fifth season hasn't been so good (at least thus far.) I wonder if this news of cancellation means ABC has made a strategic choice: will it save Invasion and let Alias slip away? (I had read that Alias would soon take Invasion's time slot following Lost on Wednesday nights, but that hardly seems to make sense now that Alias is terminated...)