I changed the heater core in my 1999 GMC Jimmy over the course of 4 days, working on it part time either before or after work (I worked 3rd shift at the time), and it was pretty much a PITA. I did it myself and flushed and cleaned the core myself which is all it needed, it wasn't leaking- just plugged. I couldn't see replacing something like that only because it was plugged as it cost more than the radiator. I also had to install a filter in the heater supply hose and ended up emptying it every 3-4 weeks of all the accumulated junk that complete cooling system flushes didn't get out.
I found our right after buying it (just a month before the flush and clean) that someone had added Ethylene Glycol antifreeze right in with the old Dexcool junk and it reacted badly.