Battery back-up?

I have a question for those on my flist who are more computer-savvy than I am. (So, that's pretty much everyone then). Should I get a battery back up for my computer? The guy from Geek Squad who came to my house in December and tried to resuscitate my old computer said it might not be a bad idea to get a battery back-up for my new computer. He said it would cost around $45. Do you all have them? If not, how do you back up your files?

Thanks.