I've been toying with DIGITS for about a week now, I'm testing both services — I have the duplicate SIM for my main line and I also have a "virtual" number that I've added to my account. The service is promising and solves a key issue, as long as you're willing to use a Galaxy device.
I love the Galaxy line, I love that they're scaling down touchwiz and I'm looking forward to the Nougat upgrade in the next month. If this is also you, and you have multiple lines you juggle, this is the solution you've been looking for. In the image above, each icon represents a different phone number being used via SMS; personal, public, and work.
But my main line is still an iPhone and the iPhone doesn't really have the greatest options. The application is alright, until you get to the messaging portion which is horrendous. It's unusable in its current graphical state in my opinion. So while it's great in concept, the end result isn't very appealing.
Its definitely worth trying either way to see where the future of mobile is headed and how a major problem that google voice attempted to solve, is finally being solved at carrier level.