Hello - Caller ID & Blocking is Facebook's Android "dialer" replacement. A lot of stock Android apps can be replaced with third party alternatives and Facebook is doing this with its own phone dialer. Hello lets you call your Facebook friends, even if they don't have a phone number.
Facebook, phone, and search in one
You have to use your Facebook account to log into Hello. On its first start up, you are choose caller ID options and how you want to address unwanted calls. While Facebook claims to block unwanted calls, the only options are to mute "commonly blocked numbers" or block hidden numbers.
Inside Hello, you can search for both contacts and businesses. Searching for businesses is very similar to Android's stock dialer, but Hello pulls information from Facebook Pages as well. Searching for general queries takes just a bit longer as it pulls relevant information from Facebook.
To call friends, simply search in Hello and their name and picture will display. You can see a snippet of information about the contact but you can also go to their full profile to see your relationship with that person.
You can call through Facebook Messenger or chat with them, but this opens a Messenger Chat Head rather than working directly inside the app. Hello pulls information from Facebook, but it also pulls information from the Google Contacts app.
Hello calling uses either Wi-Fi or your data connection so the quality can vary depending on your connection. But calling an actual phone number used the stock dialer app rather than Hello.
As a replacement dialer, Hello functions well but could use some speed improvements in search.
Diving into Facebook even more
If you communicate with friends and family through Facebook, Hello is perfect. But if you actually make phone calls, Hello is kind of useless. Any calls to an actual phone number uses the stock Android dialer. Hello acts like a phone dialer, but is more of a Facebook overlay. This means that it communicates with Facebook contacts through Messenger and searches through Facebook, but doesn't actually call phone numbers.
The fact that Hello also integrates Google Contacts information means that you're accessing information already on your phone. Another annoyance is that you don't actually text through Hello. When you want to message someone, a Messenger Chat Head pops up, adding another app to your screen. If Hello is meant to integrate Messenger, then you should be able to choose.
Android's stock dialer can also search for businesses and is much faster than Hello. This is strange because Facebook should have quicker results because it's pulling information from a smaller amount of information than Google.
In a way, Hello is like adding a screen door to a house; you add another barrier to complete a simple action. While it's nice to feel the Facebook breeze, you don't actually need it.
You don't need it
Hello could be a great if you're travelling overseas and need to communicate with friends. Calling over Wi-Fi is easy and simple, but Messenger already features calling. Furthermore, you can use WhatsApp or another chat app that includes voice calls.
With time, Hello could be a great replacement for the stock dialer. But right now, it's just bloatware.