Well, in a perfect world, both. rycanada said it best. If the feeling is "Man, we did everything we could and the dice didn't pan out" than there is still fun to be had. However, hinging everything on a single die roll (or a series of exceptionally high ones) removes the planning out and settles ONLY for the random element.
Consider Disintegrate. Up till 3.5, Disintegrate was a save or die spell. In 3.0, it was ranged touch, fort save or poof. Now, a character with a lot of hp, a descent touch AC, SR, or a great fort can survive the spell. This makes the spell more fun; a raging barbarian, a monk, or a githzerai can all avoid it or survive it. Compare to (say) Finger of Death: Fort save or death. You've now hinged on one die roll (two, SR). Only High fort PCs have a chance, one chance, to survive.