I'm ok with flashbacks...if they are adding to the story. Too often they're not necessary and not put in for the right reason. I think they need to contain some sort of essential detail that we wouldn't have been able to get another way.
Edited to add examples...examples from TV shows... whatever
So good flashbacks: Veronica Mars. They almost always added to the plot or characterization. Most often showing us a clue Veronica needed.
Bad flashbacks: well, I don't know if Dawson's Creek did flashbacks, but if they did, I bet they did them badly