I lived, worked, and had an amazing time there 1993-7. I can't agree that its atmosphere these days would remind anyone much of its darkest days in WW2. You can maybe feel echoes of that on the streets of Kazimerz (former ghetto aread) - maybe. But I'd go no further for that.
As for Warszawa - well, it was 90% destroyed in WW2. What you see now was built from the 1950s onwards. Again, I'm at a loss to see how it could resemble the wartime version.