As part of a +Action/+Cards engine, Council Room has a great ability, being the only +4 Cards in the game. The +Buy is an additional benefit, since you’ll probably be drawing enough money to need it and it saves you from needing to buy a dinky Woodcutter to get your +Buy.
Unfortunately, the card’s strength is tempered by the fact that it lets all your opponents draw a card for free. It’s still, on balance, worth it, but there are several ways to avoid helping your opponents.
One is to play Spies before the Council Room, so you can at least ensure that they draw a Victory card instead of a useful card. But this isn’t fully effective; by drawing a Victory card, they’re improving their next hand, and besides, Cellars, Vaults, and Remodels can still make use of a Victory card.
The better approach is to play a Goons after the Council Room, thus knocking them down to 3 cards. (Note that Torturer does not work, since it only involves discarding two cards, not discarding down to 3.) This can frequently wreck an opponent’s plans by forcing him to draw and discard cards he would otherwise prefer to leave on the deck.
Ideally, you would play a Goons first, then the Council Rooms, then another Goons, but this requires a ton of +Actions.
Of course, Militia also works in place of Goons, and is much cheaper as well. However, the +Buy of Council Room works nicely with Goons’ VP chip ability. Ghost Ship can also substitute for the Militia/Goons, but it’s more likely to inadvertently help your opponent by allowing him to rearrange his hands. (For instance, if the opponent has a hand of Golds, and the Council Room draws him more Golds, Militia/Goons is devastating. Ghost Ship, on the other hand, just ensures him two consecutive Province buys.)
What about Envoy?
The envoy does allow you to draw 4 cards with the caveat that your opponent selects one for you to discard. So not only is it only a net gain of three cards, with the best card eliminated by your opponent. However Envoy is still a great card because of its very cheap cost.
No, actually, he is correct that Envoy draws you four cards, since it’s draw-5-discard-1. But it’s slightly less than 4, naturally, which is why I didn’t consider it (though I should probably at least mention it).
I’ve found that this works much better with Governor/Goons.