I did mine myself, its really not hard. Just make sure the copper is dry/clean before sweating it, also soap the gas connections to check for leaks after connecting.
My next hot water heater will be one of the new units that heats the water on demand, screw the big tank!
i worked for a plumber for three years. did LOTS of water heaters. IMHO the gas connection is a LOT easier than soldering copper. gas is less than 10 psi inside the house and water is 60 - 80 psi, which is a lot harder to contain... put 7 rounds of teflon tape and coat it with pipe dope. spray with soapy water if you smell anything.