Fair call Smac and Mokesta.
In terms of Smac and Mokesta's first points, I never noticed any extra front grip after upgrading my rear bar on my FWD car, but then when pushed it chewed more fuel than I could afford at the time, so I never pushed it too hard!

So I didn't notice better drive out of a corner. I did notice that the back was less assured with a larger rear bar - particularly on turn in. It tended to skip across bumps, rather than track through them.

In terms of extra front grip from a rear bar, I found it doesn't help when you're off the gas. A sway bar does increase roll stiffness, but I'd maintain it's better to stiffen the body before the suspension. And the rear wheel can't take that extra cornering load for you when the car's pitched forward, so it's only effectively improving your balance half the time - when you're on the throttle. The rest of the time it takes it too far the other way, and, as Mokesta pointed out, it's very easy to end up like that 205GTi. Off throttle, I found a bigger rear bar just made the rear more nervous, not the front more secure. Thankfully I didn't have any experiences like AEG163!
In terms of "really useful if you've got soft suspension", I vaguely recall that Citroens and French cars in general were the first to widely adopt sway bars, so that they could maintain soft suspension but get that handling balance they're famous for. You will notice the difference on any car - even F1 has sway bars, and we all know they only use a few mm of travel. But, generally, the harder the springs, the less roll you get, the less you actually use that sway bar. The advantage of a sway bar (on the road) is that it lets you run softer springs while maintaining less roll.
I agree that oversteer's no bad thing - it's one of the greatest things in the world.

And I realise that a well setup race car's handling isn't just characterized by good grip, it's also about balance, and that a well setup car is one that can be driven with an economy of steering wheel inputs. Even rally's gone that way now - faster, but less spectacular to watch.

My point was more that I'd rather balance the car up by improving the front end grip than anything else. I think that a sway bar should only be something that you look at if you know your car very well, you've already upgraded the rest of the suspension, and your biggest problem was understeer. I guess we should answer the Bad Man's question - yes, they make quite a difference!

And, in the right hands/right place, they can be a very good thing. But I just don't think they should be the first port of call. I'd be doing negative camber bottom arms, adjustable tie rods, etc on the front, and camber/castor brackets at the back, playing with the ride height, etc, all before doing a sway bar.
In terms of transferring the load diagonally across the car to improve grip, that's something that would make it interesting to experiment with a hydro system... I'd be interested to see a setup with competition bump stops (or coil-overs with fairly light springs) and with the hydro lines connecting diagonally across the car - similar to the last Audi RS6... I'd try it myself, but unfortunately my car has been converted to dry, so the experiment would take a lot more work now...

Maybe one day...