Yes it can be a little challenging, but its worth it when you need the heat
In october of 05 i replaced my heatercore myself. you pretty much have to gut the interior, dash, gauge housing, center consel ect. a lot of little screws and bolts, If you take it to a shop they will charge you about 8 hours worth of work, i did mine in 9 so that wasnt so bad. The easy'st part is swaping out the old heatercore with the new one. then putting it back together and your done. If you do do this Good Luck
p.s. if you have air conditioning the a/c line in the engine bay may be a little rusty and there is a fitting that you will need a special tool to remove it (fuel line tool works well). The a/c line is right by your heater core hoses, or you can just cut if off if you dont use your a/c like myself-but that was just an easy way for me because i never used it.