Yeah Painkiller really has stood the test of time, many said it was a going to be a classic when it was released and the proof is there today. Many people still refer to it when they talk about "best metal albums."
I remember listening to JP before Painkiller but it was mostly singles. It was somewhere just after Painkiller that I really delved into the back catalogue and had to start buying all their albums, then when Rob left I didn't like them much.
Same thing happened for me with DIO. Until I heard Inferno Last In Live I'd really only heard his Sabbath stuff and the main singles of his solo stuff. Once I heard Inferno I just had to get into the back catalogue all the way back to Elf and one of the Elves singles.