Their fees will probably cost you more money than the money saved from tax loss harvesting. Betterment is just investing in Vanguard funds (not all of it, but definitely a few). You could spend a few days and research investing using Vanguard, but if you'd rather not do that and are willing to absorb their fees, than Betterment is a brilliant choice.

If you want to understand the basics of passive investing, The Bogleheads Guide To Investing is a brilliant book.