I use Fortigates as well - those are solid devices - @LZ good writeup in past firmware 5.0 and below Fortinet had issue with firmware upgrades, now it is very clean process. Also I would always recommend A/A or A/P configuration.

I agree with Matthew. First understand requirements and how it fits with your current organisation ( network as well as skill set). FGFW are nowdays way to go. I use fortinet good balance between cost and UTM options.