I've tried Betterment for ~1 year. Similar to Wealthfront, it's a solid choice as a "set and forget" robo-advisor with a monthly recurring deposit. One option that strongly differentiates Betterment from its competitors (namely Wealthfront) is the "tax-coordinated portfolio" with multiple accounts (at least, as of today). See https://www.betterment.com/tax-coordinated-portfolio/ for more detailed info, but in summary, they manipulate your multiple investment accounts to give your entire portfolio the "balance" of assets you want, but place individual assets into the specific portfolio that reduces the amount of overall taxes you have to pay. This feature is pretty useful for people with retirement accounts *and* other investment accounts.
Betterment is the first robo-advisor I tried (back in Dec of 2013) and I am still using it. I initially chose Betterment because it was the most established robo-advisor and trusted them with my money. I like the portfolio that Betterment invests in because It's pretty diverse although I would prefer that it had less in international ETFs. My returns have been pretty decent so far, I'm up 12.7% since December of 2013 so I'm pretty satisfied.
I think Betterment is probably the best choice if you don't mind paying a fee for your robo advisor (Wealthfront is free