I find positioning brings the most benefit. Bad positioning can mash you up when you think your winning. Bad positioning will create open files and expose your weak spots in the middle-game and end-game. When I say weak spots I mean block-in pieces and pieces that will take you 2 or more moves to protect.
Peoples thought patterns change as pieces move and get captured. This means that the puzzle changes. Therefore, flexibilty is surely an asset.
I've found that I learn more by not focusing on how many tactics I can solve, but rather on how well I can understand each puzzle.
Understanding the key ideas of the puzzle is what brings you the most benefit.
Read more about this here:
http://www.chess-strategies-tactics.com/chess-tactics/understanding-chess-tactics