We're a MSP that used to sell Juniper/Netscreen for SMBs exclusively, and as the Netscreen line started to get phased out, we evaluated five different vendors in mid-2012, and Fortinet definitely came out on top.
Today, we've got somewhere around 175 fortinet devices deployed (mostly fortigate/fortAP) and have been extremely happy with them. We really love the integration between the Fortigates and FortiAPs.
Our support team has been very helpful, and the range of additional products (UTM, FortiAnalyzer, FortiExtender, and 2FA with FortiToken in particular) have helped us look great.
I would love to see more easy "out of the box" reporting options on the Fortigate line. Higher-ups love shiny reports, and it'd be nice to be able to hand them a web usage report without going through a ton of config.