I've been doing this for over a year now, and it has proven to be very effective in our game. Almost every encounter last exactly 6 rounds, and it's allowed us to fit at least one if not two extra combat encounters in our sessions which are always short on time. I did do some things to balance it though. Minions only do 50% damage, their lifespans are so short that the increase doesn't become unfair, but it does allow them to be a threat rather than glorified difficult terrain.
The most important change, however, is that I do not double the damage on encounter or daily powers, as even if the monsters had longer life spans they wouldn't get to use them more than once. Recharge powers, on the other hand, usually have double the chance to recharge, to compensate for the monster's shorter lifespan. I'm also careful about doubling the added damage certain skirmishers and the like do (sneak attack...). I found these changes were necessary to prevent certain monsters from dropping a PC in two hits.
My player's really like it.
They know the rules ahead of time, so they avoid choosing powers and abilities designed for longer combat (regeneration, DR, & the like), which are drastically reduced in usefulness, and focus their characters on increasing their usefulness for the sort of encounters we run.
It's been quite fun, as now, because combat is so brutal, they have developed into a very coordinated team, with a good sense of how and when to nova. Seeing them recognize that an encounter is a serious threat and then respond with tactics that are equally vicious has been one of my favorite experiences as a DM. My players, for their part, really enjoy the increased encounter speed and tension because they are more dangerous. I've also found that the increase damage has made my leaders, who were less experienced players, really start to feel like they're a valued part of the team.
If you're looking for a faster & more dangerous game, I'd definitely say go for it. Just start small with encounters of the PCs level and give your PCs time to adjust before really trying to take it to them with some more dangerous monsters. Since the change, we have yet to see a TPK, but have had numerous really tense fights, where the PCs actually felt like they were threatened instead of slogs they felt sure to win we had before the change.