We currently have one and it replaced a MBR95. Both have WiFi as Wan capability so a mifi can be used as an internet gateway access device. The 1400 has greater range than the MBR95 which helps with our house setup with computers and printers and a bridge scattered in various rooms. On the road the mifi provides more than enough range for our laptops. Only reason for a router would be if we wanted to hook in a non-wireless printer and we do not print while traveling and if we did we would take a printer with 802.11 connectivity.
We do use a signal booster and external antenna to get the best possible signal strength to the Novatel 4620LE mifi and often it means the difference between having a usable signal and connecting or not.