I replaced my thermostat with a Sensi wifi thermostat and so far it's worked great. It does not require a "c" wire, it also retains control of the fan (on or auto) but you do have to figure out if you want to use the low or high fan wire unless you want to add an additional toggle switch. I love being able to have the fan on with the furnace at the same time, really helps to circulate the air.
The only down side is with no "c" wire if you access it a lot via wifi it eats the "aa" batteries pretty quickly. It takes 2, so I bought a 4 pack of rechargeable double a's and just swap them out when they get down to 1 bar.