Borrowing against a 401K is always a bad, bad choice. It's better to get a personal loan or even to use a credit card. Unless you're really in a mess -- like something medical or losing your house -- AND you have no other options, this isn't a good choice.
When you borrow against a 401K your investment isn't appreciating value; thus, you're missing out on earning interest. Yes, you're paying back interest to yourself, but -- depending on the market -- it may not be as much as you could've been earning. Also, you're paying the loan back with taxed dollars, and then in the future when you take the 401K money out to use it in your retirement, you'll be taxed AGAIN. You can find detailed explainations /examples on the internet to show just how expensive this is.
I'm shocked that so many people have taken out such loans. It just proves that so many Americans 1) aren't financially saavy, and 2) will do anything to prevent cutting back /economizing today.