I use them all the time...
Good
They are available anywhere I have Gmail access
They are in my Android and iPhone contact list
They are always synced between all devices immediately
You can search by any part of the name or email address. or physical address or phone number
I can click on a contact and go straight to SatNav (if address is entered)
Android SatNav searches in contacts
I can say "OK Google, Navigate to Bob's house" and it knows to get the address from my contacts and start satnav.
Bad
It can get messy if you autosave all email contacts
It can hold
too much information. You can get into the Google philosophy of not deleting old data.
Google may know too much about my life
Missing
Telling you which contacts you rarely use, to help with housekeeping