I love Aperture but have to admit the noise handling is not as good as other RAW conversion software. Are you using any plugin to do the job? Good news is Aperture is just months away, if the rumor Is real.
Do you mean generally, or the noise reduction tool ? If the tool, I agree. It doesn't seem to do anything at all. Then again, neither did the one in Lightroom when I last used it. Actually I have little use for NR. On the very, very rare case that I need it I have Noise Ninja. Dfine 2 seems good too. Actually very good.
What I do regret is the lack of hot pixel supression, which, for example, Iridient RAW Developer does very well. But in general the noise which the cameras I use generate at high ISO doesn't bother me.