To be honest with you, the answer is neither a yes nor a no! It actually depends! If you already own the copy of the game that you are trying to download the ROM for, then yes it can be considered legal. But, if you have never owned the respective game, then it may be illegal. Again, this is not a go-to guide as we have already said it depends from situation to situation in some cases. For instance - Most of the classic games out there are no more officially available for sale in the market, so downloading ROMs of that should probably not land you up in any trouble. Whereas in the case of emulators, you are free to download and use them as they are completely legal.