For me, the hardest part was busting loose that drain plug. Depending on where it's located, there may be some extra labor-time in removing parts to get to it. 7-years of corrosion required a breaker bar and some good tools to get that off.
Then comes the tidal wave of coolant -- make sure you have a big pan to collect it.
Otherwise, installation is simple -- screw-in the heater, run the wire, secure it, and there y'are. Add some coolant and make sure the system is free of air.
All told, it took an evening to get it all together -- not too bad.