I just want to give some clarification to some basic things regarding One Number for Multiple Devices. These were issues I could not get a strait answer on before using DIGITS myself.
1. One number for multiple devices using the DIGITS app. This is exactly like GV - you download the digits app to as many devices as you want and use the app for receiving and making calls and texts. All devices with the app will ring on incoming calls, display your number on outgoing calls, and sync your text messages. The disadvantage to this is you cannot use your native dialer or message app, you have to use the DIGITS app. This works exactly like GV and even uses a random server number for outgoing calls just like GV (but displays your number on the other persons caller id). It works very well and enables you to use your main phone number wherever you want.
2. One number for multiple devices using a Duplicate SIM card. This is just a simpler way to use your main phone number with multiple phones. It's just a duplicate SIM and its awesome. You can take your Android phone one day and your iPhone the next day (you can also do this with the above method). They both have the same SIM card (virtually speaking) so you can use your native dialer and text messaging app (although you should shut off iMessage for now and just use the Apple message app in standalone mode). It's brilliantly simple and works great. The only disadvantage to doing it this way is your text messages are not synced between your phones (unless you simultaneously use the DIGITS app but that defeats the simplicity of having duplicate SIM cards).
3. Remember with the Duplicate SIM method you do not need an additional phone line with T-Mo. It's just a second SIM card so you don't have to take the SIM out of one phone and switch it when you decide you want to carry a different phone. So you can have an Android phone and an iPhone to play with but your only paying for a single line (there will be a fee for this after BETA is over but no one knows how much).
4. With either method it appears you get HD voice, wifi calling, and all the other carrier features that you don't get with GV. So far I'm absolutely thrilled with DIGITS!