What do you like best?
The ease of use. The Sophos interface is really clean and easy to understand. You don't need very much training to be able to use. It is almost like the Mac for UTMs (I know some may not like that, but I mean that it is less console/script based set-up/admin and more UI and friendly, yet still secure).
What do you dislike?
The method of opening ports is slightly different from most other 'firewalls' and takes some getting used to, but once you understand the process and how it works it is fine, you just have to re-train your brain a little. I prefer the interface for the current models compared to the UTM 9.
Recommendations to others considering the product
Give it time. Be prepared to learn the nuances. Find a good resource for if/when you need help. We rolled it out and I have added/turned on features in stages. Their customer service is good as well. I had a unit whose HD failed and they overnighted a new unit to me. Really appreciated that to get me back up and running.
What business problems are you solving with the product? What benefits have you realized?
Protection of our systems. This is our firewall. It not only protects our network, but it acts as our content filter. We can warn users when they are about to download a filetype the could be dangerous, as well as limit or warn for certain website types that might be dangerous. We also use one for a remote office location so the VPN tunnel is of great benefit.