The players have taken advantage of this rule in a number of interesting situations. My games tend to spend much more time on exploration and role playing than on combat and other dice rolling, so not every player ends up choosing to roll the d30 every game, but so far it has been used to:Once per session each player may opt to roll the Dungeon Master’s big green d30 in lieu of whatever die or dice the situation normally calls for. The choice to roll the big green d30 must be made before any roll. The d30 cannot be rolled for generating character statistics or hit points.
- win initiative (very important when trying to get the Sleep spell off before the baddies get to attack)
- make a save-or-die saving throw vs. poison
- avoid being surprised
- attempt to pick a lock, on the reasoning that it's easier to roll between 1 and 15 on a d30 than on percentile dice
I like this house rule a lot, and the players definitely seem to enjoy it. It gives everyone an opportunity (but not a guarantee) to do something extra heroic each game, and adds a bit of strategy over when it's worth using your d30 roll. I don't feel it unbalances the game much; the game is plenty dangerous for the characters and I don't mind giving them a bit of extra luck. And that's all it is, after all - the element of chance and risk is still there.
By the way, technically the DM does count as a player and is allowed one throw of the d30 per game. I haven't used it yet, since it seems kind of unsporting on my part to stack the die rolls in the monsters' favor, but I might do someday if I had a villain or NPC that I really wanted to live to fight another day or whatnot.