Allo was launched on the Google Play Store today, and it happens to be Google's latest free messaging app, though it will not be replacing Hangouts. At the moment, Allo is only available on mobile devices and it will be linked to your main phone number. Connecting a Google account is not required, but it is required if you want the Google Assistant to learn more about you and give you better, personalized results when using the messaging service. Just like WhatsApp, Allo cannot be used on multiple mobile devices at the same time which might be a hassle for individuals who frequently switch between devices, but the same Allo account can be used on a particular device even if SIM cards are swapped. This can be pretty useful for travelers who buy local SIM cards at destinations but want to continue using Allo, all without incurring extra roaming charges from carriers.
Changing the SIM card on your device will not end your previous session on Allo, the same way it works on WhatsApp. So even if you swap SIM cards overseas, you will be able to use your current Allo account as per usual. In order to do this, you will have to activate your Allo account with your main number using SMS authentication. Once this is done, you can purchase a local SIM card at your destination and insert it into your device. You will then be able to use your Allo account which is tied to your main number, and you can make use of the local SIM card for data, texts and calls, all without expensive roaming charges. Do take note that this method will work as long you avoid logging in to Allo on a different device with your main number.
Since Allo is an app still in its infancy, there are bound to be some bugs in the app. On top of that, certain features which are usually found in a typical messaging app might not be there. However, the team working on Allo will most likely add these features into the app in the near future.