Is buying a membership to download PS3 games online beneficial?

I have always wondered if the games I buy could just be bought online and downloaded to my PS3 so I could save a trip. Is the gam quality the same as a normal disk game? Does the picture look the same? The sound? Is it exactly the same as playing a regular disk game? Has anyone tried it before to give some feedback of any kind'? Also what sites could be used if it is just as good as playing a regular disk game? Thanks so much, sorry for all the questions!