I am looking into putting a Firewall in front of our web servers. But we just recently implemented new next-gen firewalls. Does it make sense to put a WAF in front of our web servers as well?
The answer to this question would be reliant on the capabilities of your 'nex-gen' firewall. If your ng-firewall offers the ability to perform deep packet inspection and detect and prevent web application vulnerabilities, you need to figure what the delta is for the ng-firewall with a waf. If both offers the same capabilities and have no significant delta, addition of WAF could be an unnecessary expense.
That being said, WAF are generally built with a specific intention - Look at the application traffic and prevent application specific attacks. It is likely to serve better for that puspise than a nex-gen firewall for that purpose.