flashbacks are very tricky. ANY flashback to something we actually saw already, or were already told without flashback, pisses me off. It's a waste of space and slows down progress.
Flashback that actually gives you something new can be alright, if it's something interesting like a character's past. However if it drags on too long (for example a few pages of manga being 2 - 3 anime episodes) then I still get annoyed.