For 401k plans that have access to Vanguard's target date funds, they can be a decent choice. Unfortunately, the majority of 401k's don't even offer Vanguard as an option -- instead, employees are stuck with high-fee mutual funds because they give kickbacks to the 401k company.

That said, employees using our investment advice get additional diversification (6-7 asset classes), as well as a more personalized recommendation that takes into account not just your age, but also your risk tolerance, financial situation, other investments, etc. We think that's better than a one-size-fits-all fund.