For the un-itiated, 'offset keys' is a technical approach in CG to try and achieve an organic solution of weight. The basic idea of the offset-keys approach is as follows: The character moves from frame 1 to frame 10, frame 10 being the pose you want to move into.
The offset-keys technique in theory is very logical. The idea is that if the hips hit at frame 10, then if the mid-spine hits at 11, the up-spine hits at 12 and the head hits at 13 then you'll get good overlap.
Like I said, logical in theory. But weak in practice. At best - when the technique works perfectly- all you get is some overlap at the end of the move, but you get absolutely no drag in the middle of the move. Drag during the move is where the weight comes from. Simply overlapping parts at the end of the move is not enough- it's a cheap trick that doesn't solve the problem. And the technique rarely works perfectly. Due to the way each object in CG works mathematically you end up with all kinds of unintended consequences when you use the offset method. The most common result is a funky hitch in the motion due to the way the math mixes at a given frame for the combination of controllers. (compare this non-offset animation to this one that uses offset keys. Note the junky bits of motion.) These unintended consequences make cleaning up the move much harder and often leads to the animator setting a ton of extra keys all over, tweaking a bunch of f-curves, making dozens of playBlast previews all in the effort to get rid of the hitches in the motion. The animator's energy is spent on removing motion problems introduced by the offset-keys trick, not on building good animation.
The key to good weight in a move is to get a sense of drag during the move. This is done not by some math trick but by making the changing shapes work as the character moves. When combined with some overlap at the end of the move things look much better and are a lot easier to clean up. If there's one thing CG animators need to do it is to start thinking about changing shapes in motion and to stop thinking in terms of moving parts. Moving parts are technical. Changing shapes are organic. All animation - regardless of media - is a 2d image on a screen. CG animators just use a puppet to create those visual shapes on a 2d plane. The visual impression of shapes changing in 2d hand-drawn animation will work in CG. We need to shift our thinking to how to make those shapes work for us. The key comes back again to making good breakdowns. Breakdowns are the secret to the shapes, the secret to getting clean motion with weight.
Here's a quick visual that might help.
(I have my theories as to why). The result is the same weightless movement we associate with CG.
Really the problem is all in our heads.
UPDATE: Commenter Jim notes that offsetting keys is very useful in the polishing stage of a shot. I agree. At the final polish stage I offset keys quite a bit, sometimes on a per channel basis. Key offsetting is a perfectly valid technique as a tool for polishing the motion because at that point it's all fine detail modification. However key offsets are unsuited as a technique for building the fundamental motion to begin with.