Betterment Advantages: 

Good for lazy investors, time-restricted investors, investors with little knowledge, investors who want a hybrid of advice and tools without paying the crazy fees of mutual funds or an advisor.

Tools are great, although they can be found online
Portfolios & Rebalancing are not terrible
Cost is relatively cheap when compared to a mutual fund, advisor or active trader.

The biggest advantage, is that it is easy, quick and convenient

Disadvantages: 

Limited by portfolios offered
Lack of data for performance in a bear market
Still relatively new, things are being developed/ironed out
If you have the time and knowledge, this might not be for you