Is Game Modding all it's cracked Up to be?

i am considering buying a hardrive data migration kit so i will be able to mod certian games (mostly GTA IV, red dead redemption, ext. NOT call of duty games)....I was just wondering if its worth all the trouble of doing it,...whats it really like? Youtube makes it seem cool (you can get better weapons, real cars, tanks, jetpacks, ext.) but is it really all that great. If you think it is, could you please recomend the best site to get the mods at? Thanks.