I use it for my online game - a mix of Call of Cthulhu, Dark Matter, and Delta Green, set in 1760s Boston. I find it works pretty well for what I'm trying to do, which is create an environment in which the PCs feel like combat is something that they should only engage in as a last resort, something that is threatening to their lives pretty much any time they engage in it.
The rules do take some getting used to, particularly if you haven't played much besides D&D. Several basic assumptions are pretty different. Your players will likely have trouble remembering things for a while, and will occasionally react as if they were still playing regular D&D, resulting in some pain.
It does take some tweaking to get the system to work with magic well. I've largely avoided direct-damage magic as a part of my setting, which helped me out quite a bit, but probably won't be a useful suggestion for somebody who's intending to use it for more 'mainstream' D&D (if that's even what you're after - what are you going for?) If you use magic as written, spellcasters are EXTREMELY overpowered, as they deal lethal damage to just about any Large or smaller creature with just a spell or two. I found that rewriting the "Maximum Damage by Spell Level" table worked pretty well, though it takes some time to figure out just where you want it.
I haven't found that it bogged down combat much. At least, not once you get used to it. Once everything's figured out and statted out, things run about as quickly as regular D&D. Opposed Defense rolls are something I dropped fairly quickly; I did find that those bogged down combat a bit too much for my tastes. Penalties from damage will irk your players for a while, at least if they're like mine.
As far as making death a reality... well, that's more or less up to you. It does pretty well hose the existing CR system, in my experience. A creature that deals a lot of damage will be VERY powerful in G&G. Ogres, trolls, most predatory animals, etc become tougher opponents. Anything with a high natural armor bonus (over about seven or eight) becomes quite hard to deal with, as it reduces all physical damage by that amount. Try fighting a creature with DR 10/- some time, and see how well the melee fighters do against it. Of course, the PCs will likely have tanks of their own, which means that combat has the potential to last longer.
I will say that I've had no PC fatalaties in my current game, which has been going on for about seven months now. Of course, I can list all the combats that we've been involved with during that time - it's less than ten, I know. Remember, CoC is a major influence on this campaign - if they'd been in a lot of fights, they'd be dead. And I do think the Grim and Gritty stuff helps inject that into the game.
That help to answer your question? Or was it just a mess of pointless rambling? I suspect the latter, but then, it's four in the morning. Ah well. If you have any more questions, feel free to ask.
