Here are a couple of good articles that point out there are times when you should NOT participate in your employers 401(k) plan. SmartMoney article. Ask Holly article.
If your employer's 401(k) plan:
(1) offers no matching funds,
(2) charges a commission on your purchases (also called a front-end load, or sales charge),
you should opt NOT to participate in your 401(k). Instead, fund an IRA or Roth IRA.
If your 401(k) plan charges a commission on purchases, but does offer matching funds, you should only contribute enough to obtain the maximum employer matching funds and no more.