I wouldn't advise you to use it unless it's a necessity, which sometimes might be the case with you, as you're working on an existing project. But try to stay away from it and use as little !important as possible.
The problem is if you use too many of them then one might inadvertently overwrite the other. Your code is also much less legible and anyone who comes after you to maintain this will tear his/her hair apart, as trying to find !important every time you do something in the CSS is a royal pain.
Check this: http://webdesign.about.com/od/css/f/blcssfaqimportn.htm
