Before I knew of this product line (thanks to FortiGuru), I was looking at the following vendors:
Fatpipe (because they started the BGP alternative movement)
Mushroom Networks

Does anyone have any experience with these other devices and can give me their perspectives on how FortiWAN might be a better solution (or worse...)?

Currently my lean is towards Mushroom Truffle and Truffle Lite due to ease of setup, friendly support that will give you their time (was on the phone with them for an hour), and great pricing (out the door from under $2k to mid $2k) as a reseller.

Thanks in advance.