Hello,
I currently use Betterment and have been for about two years. I have enjoyed it--it's really simple, I believe in their investment strategy long-term (they have a heavier mix of Developed and Emerging stocks/bonds) and whenever I've had a question, they've been really responsive and helpful.
Lots and lots of people here use Vanguard, and for good reason, they're the cheapest. But I do like having Betterment do the investing, the rebalancing, the re-investing dividends, etc. and the small fee (which to some on these forums is abhorrent) is worth it to me.
Also, our great leader MMM uses Betterment. So that's a good endorsement too!