All my comics so far have been in B&W, but I'm switching to coloring for my next, long story for several reasons.
First off, people, especially on the web, like colors: printed comics can get away with B&W, but let's face it, you likely turn off a large number of potential readers online if you don't have catchy colors.
Second, it's a challenge: I want to learn how to color, because I want my art to shine as much as possible in my eyes as well as my readers' eyes. Perfecting yourself means constantly pushing your boundaries and breaking through your limits!
Finally, I am now at a point where I am kind of satisfied with my line art, and I believe that coloring will not cover its defects, but bring out its quality. I really dislike comics with shiny colors that just cover up line art flaws. I don't want mine to end up like that! 
So, yes, coloring adds to production time, but I think it's really worth it most of the time! Just pace yourself 