How well a car idles (and possibly very low speed driveability) is determined by the injector as well. Modified ones can idle rough and run a little rough down low I believe, but check on the latter. It also depends on how anal you are as far as what constitutes a smooth idle.

According to Scott Siegel, Nismo/Tecx (sp?) are the best followed I believe by Power Enterprise then modified stockers. Iirc he said stay away from Helix like the plague. If that is the correct manufacturer, some tuners won't even tune a car with 'em because they are so hard to get running smoothly.