Has anyone tried these variant rules from the d20 SRD? Basically, the players roll everything they usually do, plus their own "defense checks" against enemy attacks and "magic attacks" for things that require enemies to make saving throws. In both cases, the monster's side of the roll is converted into a flat value.
I haven't tried it yet, but my first criticism is that it doesn't actually save the DM any time at all. He or she still needs to pay attention to every roll unless he just gives the players the monsters' relevant modifiers to calculate for him. The variant ruleset also says nothing about the hidden rolls a DM sometimes needs to make, like when a player doesn't necessarily know his or another character is making a check, or even when everyone knows when a check is being made but the results of which may not be immediately obvious.