I put one on both of my Vibes for a total of 9 years use. To me they make a huge difference. Much less body roll and understeer. I bought the one made by Progress.
Keeps the car from rolling or tilting in the round parts. curves I was hesitant to take at moderate speeds, I can now easily take at posted speeds, and perhaps higher speeds under the right situations.
I *think* that the only sway bars currently available for the Vibe ('03-'08) is the front and rear set sold by Toyota Racing Development (TRD) and the rear bar sold by Progress. It is my understanding that the TRD bars are/were made by Hotchkis, but that they no longer market the bars themselves. Whether they still make them for TRD and simply won't sell the bars themselves due to a non-competition contract, or whether they have ceased making them and TRD is simply selling existing stock, I do not know.
I installed the TRD bar set (see my thread a few below this one) and they work great. The car corners flat now, but I suspect most of this is due to the heavier front bar. I drove the car once after installing the front bar, but before installing the rear bar, and I really cannot tell much difference between the TRD front/stock rear, and the TRD front and rear.
After reading the revived thread on the rear bar loosening, I'll have to keep an eye on it. I'll put a torque wrench on the bolt heads next oil change. Don't want to turn them and break the locktite loose, but need to pull enough torque to be sure they are not working loose.
As far as ride quality, the sway bars only affect handling in turns. Traveling straight down a smooth level road, the sway bars are just along for the ride and have no affect on ride quality.