At Network Thinking Solutions, we have been using OpenDNS for over a year now. A little history, we have and still use Kaspersky and MalwareBytes as setup and offered using Kaseya. The combination of these two products has been very good and limiting virus/malware issues for our clients for many years now.
On occasion we still had problems with virus and malware and started investigating other solutions to complement what we were using. Our firewalls have virus/malware protection but that seemed to rarely help. I came upon some reviews of OpenDNS and it sounded positive. Basically, you point all of your clients to use the OpenDNS servers for name resolution. You can do that in several different ways such as a local agent, having your servers use OpenDNS as their lookup and lastly using their VMWare appliance. We have used all three of these options in one shape or form over the year depending on the client and their needs.
What have we found? It works great. Virus/malware issues are very rare now and usually very easy to clean. OpenDNS blocks users from accessing known botnets and malware infected sites. I do not have hard numbers but we used to have 5-10 infections per month and I would say we are down to 3-4 and this is usually for clients we do not have OpenDNS backing up Kaspersky and Malwarebytes. Setup is pretty easy and well worth it. I would highly recommend the product. We currently use their MSP package.
My only hesitation when I went with this product was the implication it was freeware and not enterprise ready. After seeing how this works, I could have not been more wrong. The products is rock solid and their VMWare appliance is great. Oh by the way, Cisco just bought them this month. I think that gives them gravitas they were missing.