, they do work. It's a small difference so it is hard to notice - about 5hp. The main thing they do is improve gas mileage (a proven fact) and move your torque and HP curve down lower where you can more easily use it. Because of this it doesn't actually make much more hp, however, it makes your jeep faster.
I would suggest swapping to a '99+ intake manifold, it is redesigned and it's like it has a TB spacer built in. It also has better designed runners....it's good for 10+hp and cost about the same as a TB spacer. The "swirl" that is machined into some TB spacers that claims to make even more power is worthless, so don't pay any extra for that feature.