I have always found that plans are useless, but planning is indispensable.
Eisenhower was making this comment in a military context, and it fits in well with Helmuth von Moltke’s much earlier quote:
No plan of battle ever survives contact with the enemy.
If we remove the quotes from their military context (although I have encountered plenty of people in my career who can legitimately lay claim to being “the enemy”), they talk about the usefulness of the planning process itself. A good project plan is rarely “useless”, but the planning that went into it should have brought many different aspects of the problem into play, and should have led to a far greater understanding of the problem space. This understanding helps when the plan makes contact with “the enemy” and has to be rethought.
I refer back to this post where the answer to the question “what was the most useful information you recorded during those design meetings?” was “the reasons why we didn’t do something”.
The two quotes also show the differences between “traditional” design processes and Agile design processes. Up front planning absolutely has its uses, but the plan inevitably has to change when you start implementing it.