Documentation

https://help.salesforce.com/servlet/servlet.FileDownload?file=015300000035wMDAAY

First Impressions

Good Things

Things to Know

Reuse

Object

Emails

Activating

Issues

Lots of known issues https://success.salesforce.com/issues_index?tag=Process%20Builder

Building a Process

It is really hard to see exactly what the criteria are, and you have to click into the diamond to see it - BEWARE! mistakes are going to be made. 

I had to really shorten the Criteria Name so it displays in the diamond in a readable fashion. 
The 3 fields are actually filters on the Campaign - not that you can tell that.

I suggest you use Formulas instead - it is at least somewhat readable. 

Updates

Fields

Using the fields selector has a few issues

Emails

Chatter

Formulas

Auditing