It is never legal...always illegal no matter what anyone says...
^Yep, he is right. Just downloading a rom is illegal, there is no "24 hour rule" or owning the game. No matter what you do you are infringing a copyright.
Anyways... I still use roms, they are great for hard-to-find games. I'm a big SNES fan so most of my roms are for that system.