All a throttle body spacer does is move the engines peak power delivery. (the powerband.)
It will work better in conjuction with other mods like others have said, though.
With that being said, my dad has one on his Grand Cherokee with the 4.0.
Other than a K&N FIPK and Flowmaster 40 on it's pretty much stock.He said he noticed a small difference but nothing major. For the $70 or so dollars though, what do you expect.
Expect a nice whistleing sound at certain RPM's also. Most do it, I've never really found it annoying, sounds kind of cool actually.