They are a combination of Vanguard's big total market index funds, they automatically rebalance, and get more conservative over time.
They are fine in a 401k plan or IRA where taxes on trades aren't a consideration. However, keep them out of taxable accounts as their automatic rebalancing can create capital gains distributions.
In addition, they are slightly more expensive than buying the underlying funds individually, but they rebalance for you. If you plan on rebalancing yourself, buy the funds. If you aren't sure you would rebalance when you need to, like in a down market, then the automatic rebalancing is worth it.